Semiconductor integrated circuit device and method of activating the same

ABSTRACT

A dynamic RAM is divided into an input circuit block responsive to an input signal supplied from an external terminal, inclusive of an operation start signal, an internal circuit block activated in response to the signal inputted from the input circuit block, and an output circuit block for outputting a signal outputted from the internal circuit block to an external terminal. A plurality of switch MOSFETs are provided in parallel between a power line for applying an operating voltage supplied from an external terminal and an internal power line for a first circuit portion in the internal circuit block, which does not need a storage operation upon reaching its non-operating state. Further, the switch MOSFETs are stepwise turned on in response to controls signals produced by delaying a start signal supplied through the input circuit block in turn, so as to perform the supply of each operating voltage.

The present application is a continuation of Ser. No. 08/762,883, filedDec. 12, 1996, U.S. Pat. No. 5,724,297, the entire disclosure of whichis hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor integrated circuitdevice and a method of operating it, and principally to a techniqueeffective for use in a digital integrated circuit device such as adynamic RAM (Random Access Memory) comprising CMOS circuits eachcomposed of low threshold-voltage type MOSFETs and in a method ofoperating the digital integrated circuit device.

MOSFETs experience reduced withstand voltage with increasedmicronization. It is therefore necessary to reduce the operating voltageof a circuit composed of the MOSFETs shaped in micro form. Since a gatevoltage supplied to the gate of each MOSFET is also lowered in thiscase, it is necessary to reduce the threshold voltage of the MOSFET sothat even the lowered gate voltage provides for flow of a desiredcurrent. However, when the threshold voltage is reduced, a leakagecurrent (hereinafter called a "subthreshold leakage current"), whichflows when each MOSFET is brought into an off state, in which the gateand source thereof are equal in voltage to each other, increasesexponentially. Thus, even in the case of a CMOS circuit, currentconsumption at the time of its deactivation increases.

A circuit for reducing the subthreshold leakage current referred toabove has been disclosed in Japanese Patent Application Laid-Open Nos.6(1994)-237164 and 8(1996)-83487 and U.S. Pat. Nos. 5,274,601 and5,408,144 by way of illustrative example. As a method of reducing theleakage current using such a circuit, a CMOS inverter circuit wherein atthe time that an input thereof received during its non-operation and anoutput thereof have been determined as a high level and a low level,respectively, will be described by way of example. In this case, a Pchannel MOSFET of the CMOS inverter circuit is in an off state and an Nchannel MOSFET thereof is in an on state. A leakage current produced inthe CMOS inverter circuit is determined depending on the subthresholdleakage current of the turned-off P channel MOSFET.

A P channel power switch MOSFET is provided between an operating voltagenode connected to the source of the P channel MOSFET of the CMOSinverter circuit and a power line and is turned off upon non-operation.In doing so, the potential at each internal power line placed in afloating state is reduced by the subthreshold leakage current. When thepotential is reduced to a some extent, a reverse bias voltage is appliedbetween the gate and source of the P channel MOSFET of the CMOS circuitso that the subthreshold leakage current can be substantiallyeliminated.

SUMMARY OF THE INVENTION

The inventors of the present application have discussed the applicationof a method of reducing a subthreshold leakage current to a dynamic RAM.In this case, the present inventors have found various problems whichneed to be solved to avoid sacrificing the operating speed of thedynamic RAM and to effectively reduce the subthreshold leakage current.Namely, an internal power switch MOSFET is turned off upon standby toreduce the subthreshold leakage current and is turned on upon memoryaccess. In doing so, a pulse-shaped large current will flow when acontrol signal for changing such a MOSFET from the off state to the onstate rises and the power node of the internal circuit is charged up inresponse to the turning on of the MOSFET. This pulsating current willincrease the value of the peak current of which a semiconductorintegrated circuit device. Upon mounting of such a system, the currentcapacity of the power device must be increased so as to correspond tothe peak value.

The increase in the circuit function and circuit scale of thesemiconductor integrated circuit device and the reduction in its sourcevoltage with the device micronization as described above tends toward asize reduction of a system including the device such as a portableelectronic device or the like. A battery is also expected to beinevitably used as the power supply. However, the increase in peakcurrent offers a large problem as viewed from the power device of thesystem, which needs its size reduced. Even in the case of thesemiconductor integrated circuit device, large noise is produced in thepower line with the occurrence of the peak current referred to above,and hence the operating margin thereof is made worse.

An object of the present invention is to provide a semiconductorintegrated circuit device which is capable of realizing less powerconsumption while ensuring its operating margin. Another object of thepresent invention is to provide a semiconductor integrated circuitdevice capable of realizing high integration, a voltage reduction andless power consumption without sacrificing its operating speed.

The above and other objects, novel features and advantages of thepresent invention will become apparent from the following descriptionand the appended claims of the present specification, taken inconjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

A summary of a typical one of the inventive features disclosed in thepresent application will be described in brief as follows. A pluralityof switch MOSFETs are provided in parallel between internal power linesfor a plurality of circuit blocks divided for respective functions andrespectively set so as to perform circuit operations in response tooperation control signals and a power line for delivering an operatingvoltage supplied from an external terminal. These switch MOSFETs areturned on in domino or stepwise in response to control signals producedby successively delaying the operation control signals, so as to providethe supply of operating voltages.

A summary of another typical one of the inventive features disclosed inthe present application will be described in brief as follows. A dynamicRAM is divided into an input circuit block responsive to an input signalsupplied from an external terminal, inclusive of an operation startsignal, an internal circuit block activated in response to the signalinputted from the input circuit block, and an output circuit block foroutputting a signal outputted from the internal circuit block to anexternal terminal. A plurality of switch MOSFETs are provided inparallel form between a power line for applying an operating voltagesupplied from an external terminal and an internal power line for afirst circuit portion in the internal circuit block, which does not needa storage operation upon reaching its non-operating state. Further, theswitch MOSFETs are turned on in domino or stepwise fashion in responseto control signals produced by delaying a start signal supplied throughthe input circuit block in turn, so as to perform the supply of eachoperating voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings wherein:

FIG. 1 is a block diagram principally showing examples of an input unit,an X-system circuit and an array block employed in a dynamic RAM towhich the present invention is applied;

FIG. 2 is a block diagram principally illustrating examples of aY-system and write circuit and an output buffer employed in the dynamicRAM to which the present invention is applied;

FIG. 3 is a circuit diagram showing one example of an X-system addressinput unit employed in the dynamic RAM to which the present invention isapplied;

FIG. 4 is a circuit diagram depicting one example of a predecodersupplied with an internal address signal, which is employed in thedynamic RAM to which the present invention is applied;

FIG. 5 is a circuit diagram specifically showing examples of an Xdecoder, and a latch circuit and a word driver connected thereto, whichare employed in the dynamic RAM to which the present invention isapplied;

FIG. 6 is a circuit diagram illustrating one example of a mat controlcircuit employed in the dynamic RAM to which the present invention isapplied;

FIG. 7 is a timing chart for describing one example of the operation ofthe dynamic RAM to which the present invention is applied;

FIG. 8 is a block diagram showing examples of a memory array and itsperipheral circuits employed in the dynamic RAM to which the presentinvention is applied;

FIG. 9 is a block diagram depicting examples of an input/outputinterface and a timing control circuit employed in the dynamic RAM towhich the present invention is applied;

FIG. 10 is a fragmentary circuit diagram showing one example of a memoryarray employed in the dynamic RAM according to the present invention;

FIG. 11 is a cross-sectional view showing, as one example, a devicestructure for describing the dynamic RAM according to the presentinvention;

FIG. 12 is a block diagram for describing one embodiment of asemiconductor integrated circuit device according to the presentinvention;

FIG. 13 is a block diagram for explaining another embodiment of thesemiconductor integrated circuit device according to the presentinvention;

FIG. 14 is a circuit diagram showing one example of an X-system inputunit employed in the dynamic RAM according to the present invention;

FIG. 15 is a timing chart for describing one example of the operation ofthe X-system input unit shown in FIG. 14;

FIGS. 16A and 6B are respectively schematic structural sectional viewsshowing examples of MOSFETs employed in the semiconductor integratedcircuit device according to the present invention;

FIG. 17 is a characteristic diagram illustrating the relationshipbetween a gate length of an N channel MOSFET and its threshold voltageto describe the present invention;

FIG. 18 is a characteristic diagram showing the relationship between apeak current, a delay time of a switch MOSFET start signal and the like;

FIG. 19 is a circuit diagram illustrating another embodiment of thepresent invention;

FIG. 20 is a timing chart for describing the operation of the embodimentshown in FIG. 19;

FIG. 21 is a system block diagram showing one example of a one-chipmicrocomputer to which the present invention is applied; and

FIG. 22 is a circuit diagram illustrating a portion of the circuit shownin FIG. 14 formed by MOSFETs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will hereinafter bedescribed with reference to the accompanying drawings.

FIGS. 1 and 2 are block diagrams showing one embodiment of a dynamic RAMto which the present invention is applied. FIG. 1 principallyillustrates an input unit, an X-system circuit and an array block. FIG.2 shows a Y-system and write circuit and an output buffer. In the samedrawings, signal transfer paths employed in the present dynamic RAM donot faithfully correspond to signal transfer paths as described in thenormal circuit blocks to provide easy understanding of the presentinvention. Further, the same drawings are mainly plotted from theviewpoint of the supply of an operating voltage to each circuit block.

The dynamic RAM according to the present embodiment is roughly dividedinto circuits each placed in a state of being supplied with power at alltimes from the relationship with the outside, for example, as in aninput unit and an output (circuit) unit, such as an output buffer or thelike, and internal circuits other than the circuits referred to above.Therefore, the respective circuits, which constitute the above-describedinput unit, and an output circuit typified by an output buffer and acircuit having the need for a memory operation, of the internalcircuits, are respectively electrically connected to a source voltageVCC supplied from an external terminal and a circuit ground potentialVSS.

On the other hand, each internal circuit for forming a low-level outputsignal when a memory in a CMOS circuit is in a non-operating state, inother words, in a standby state to reduce a subthreshold leakagecurrent, has a source voltage side electrically connected to sub powerlines or sub voltage interconnections or wires (first internal powerline). Further, each internal circuit for forming a high-level outputsignal has a ground side electrically connected to subground lines(second internal power line).

In the present embodiment, the internal circuits are roughly classifiedinto a X-system circuit and a Y-system circuit to reduce a peak currentat the time of the supply of the voltage to each above-describedinternal power line without sacrificing a substantial operating speed.This is because they are different in operating timing from each other.Further, the X-system circuit is further separated into two, includingan X-system circuit for forming a word line select signal and a portion(circuit portion) provided within the array block, for forming aword-line select signal.

The sub power lines are classified into VCTX, VCTA and VCTY according tothe division of the internal blocks as described above, and thesubground lines are separated into VSTX, VSTA and VSTY. Although notrestricted in particular, a plurality of P channel switches MOSFETS QP1and QP2, and QP3 and QP4 are respectively provided in parallel betweenthe sub power line VCTX and the power line VCC and between the sub powerline VCTA and the power line VCC. Although not limited in particular, aplurality of N channel switches MOSFETs QN1 and QN2, and QN3 and QN4 arerespectively provided in parallel between the subground line VSTX andthe ground line (first main voltage interconnection or wire) VSS andbetween the subground line VSTA and the ground line VSS.

These P channel and N channel switches MOSFETs divided into pairs arerespectively supplied with control signals φXB and φX, and φAB and φA.These control signals φXB and φX, and φAB and φA are shifted in timing.The control signals φXB and φX are generated in relatively quick timingin association with their operation sequences. The control signals φABand φA are generated with a relatively slow timing.

The control signal φXB is not supplied common to the gates of the Pchannel switches MOSFETs QP1 and QP2, which are provided between the subpower line VCTX and the power line (second main voltage interconnectionor wire) VCC associated with the above-described X-system circuit andwhich are connected to one another in parallel. Namely, the controlsignal φXB is supplied to the MOSFET QP1 corresponding to the input sideand a signal obtained by delaying the same control signal through adelay circuit (control circuit) 17a is supplied to the MOSFET QP2corresponding to the output side. In the same drawing, the two switchesMOSFETs QP1 and QP2 are typically illustrated by way of example.However, the X-system circuit is composed of multistage logic circuitswhich constitute an X predecoder 6, a mat select circuit 7, an X addresscomparator 8 for making a comparison between redundant addresses, a matcontrol circuit 9, etc.

The sub power line VCTX for supplying an operating voltage to theselogic circuits extends along a circuit area where it is formed.Therefore, a number of switch MOSFETs such as the switches MOSFETs QP1and QP2 are provided in parallel form between the sub power line VCTXand the power line VCC so as to provide a desired current supply abilityby their composite conductance. In other words, one switch MOSFET isformed into a relatively small size in such a manner that the currentsupply ability required to activate the X-system circuit can be sharedbetween the plurality of MOSFETs and realized by them.

In the same manner as described above, the control signal φX is notsupplied common to the gates of the N channel switches MOSFETs QN1 andQN2, which are provided between the subground line VSTX and the groundline VSS associated with the above-described X-system circuit and whichare connected to one another in parallel. Namely, the control signal φXis supplied to the MOSFET QN1 corresponding to the input side and asignal obtained by delaying the same control signal through a delaycircuit 17c is supplied to the MOSFET QN2 corresponding to the outputside. Even in the case of the switches MOSFETs QN1 and QN2, in a mannersimilar to the MOSFETs QP1 and QP2, a number of MOSFETs are provided inparallel between the subground line VSTX and the ground line VSS so asto provide a desired current supply ability by their compositeconductance.

Such a switch MOSFET division can bring about the following advantages.One of them is that since the switches MOSFETs are respectivelydispersed and formed between the power line VCC and the sub power lineVCTX and between the subground line VSTX and the ground line VSS, thedegree of freedom of the layout of these MOSFETs can be increased.Namely, the degree of freedom thereof can be realized by suitablyproviding relatively small MOSFETS within spaces defined between theformer two lines and between the latter two lines. By successivelyactivating these MOSFETs in a domino mode, they can be directly drivenby relatively small inverter circuits or inverters respectivelyconstituting the delay circuits 17a and 17c, so that a drive currentsupplied to the gate of each switch MOSFET is dispersed so as to controlor suppress the peak current.

Similarly, since the switches MOSFETs are reduced in size, the value ofcurrent that flows when each switch MOSFET is turned on, is renderedrelatively small, and the switches MOSFETs are successively turned on inthe domino mode, the current, which flows in each internal circuit inthe X-system circuit, can be also dispersed on a time basis so as tosuppress the peak current. By determining the order of activating theMOSFETs in the domino mode in line with a signal transfer mode, thesignal can be transferred with satisfactory efficiency by less currentas will be described later.

The P channel switches MOSFETs QP3 and QP4 provided between the subpower line VCTA and the power line VCC are provided so as to correspondto the array block, and the N channel switches MOSFETs QN3 and QN4provided between the subground line VSTA and the ground line VSS arealso configured in the same manner as described above. Further, theswitches MOSFETs QP3 and QP4 and QN3 and QN4 are switch-controlled on astepwise basis by the control signals φAB and φA generated with a delay,the delay signals being produced by delay circuits 17b and 17d.

The array block comprises an X decoder 12, a memory array 15, a worddriver 13 and a sense amplifier 14. One memory mat is composed of acombination of the memory array 15, the X decoder 12 and the senseamplifier 14 and hence a plurality of memory mats are provided as awhole. Therefore, the X decoder 12 associated with a memory mat selectedby the mat control circuit is activated to thereby select a pair of wordlines in its corresponding memory array 15. Thereafter, the storedinformation read into a pair of bit lines by the selection of the pairof word lines is amplified by the corresponding sense amplifier 14.

In the present embodiment, in order to ensure a relatively large currentrequired to perform the operation of amplification by each senseamplifier, a common source switch 16 for forming a signal for activatingthe sense amplifier is not electrically connected to the sub power lineVCTA and the subground line VSTA described above, but is electricallyconnected directly to the power line VCC and the ground line VSS. Thisis similar even to the output buffer which needs the flow of a largeoutput current.

The sub power line VCTY and the subground line VSTY are provided so asto correspond to the Y-system and write circuit. Although not limited inparticular, a single P channel switch MOSFET QP5 is provided between thesub power line VCTY and the power line VCC. Although not restricted inparticular, a single N channel switch MOSFET QN5 is provided between thesubground line VSTY and the ground line VSS. Each of the switch MOSFETsQP5 and QN5 is formed in a relatively large size so as to provide theflow of current required to activate the Y-system and write circuit.

However, control signals φYB and φY are set so that their rising edgesare unsharpened to suppress a peak current for driving each switchMOSFET and a peak current at the time that each switch MOSFET is turnedon. As the simplest method, there may be employed a method of formingthe control signals φYB and φY by a drive circuit, such as an inverteror the like, having only a small conductance sufficient to increase thetime constant of a gate capacitor of each of the switches MOSFETs QP5and QN5 formed into relatively large sizes.

Since the current required to vary a gate voltage supplied to each ofthe gates of the MOSFETs QP5 and QN5 is less and the MOSFETs QP5 and QN5are gently brought into an on state owing to the adoption of such aconstruction, the peak value of the current supplied to each of the subpower line VCTY and the subground line VSTY can be suppressed. Thus,when the above construction is applied to the Y-system circuit, arelatively long time exists before the Y-system circuit is activatedsince a low address strobe signal RASB is rendered low in level andthereby a memory access is started. Therefore, the switches MOSFETs canbe set so as to have a current supply ability necessary for the aboveoperation after the elapse of a desired time in such a simpleconfiguration that each power switch MOSFET is driven by an invertercircuit having a low current supply ability.

In the Y-system and write circuit, an address signal variation detectorATD detects a change in Y address signal and starts an equalizing signalgenerator 26 and a main amplifier control circuit so as to equalize aninput node of a main amplifier and control the operation ofamplification of the main amplifier. Each circuit block YB1 iselectrically connected directly to the power line VCC and the groundline VSS without being electrically connected to the sub power line VCTYand the subground line VSTY to stabilize its operation.

Other circuit blocks in the Y-system and write circuit are electricallyconnected to the sub power line VCTY and the subground line VSTY. Ofthese circuits, reference numerals 28, 33, 29, 30, 31, 32, 34 and 36respectively indicate a Y predecoder, a Y decoder, a Y addresscomparator for making a comparison between redundant addresses, a mainamplifier, a write buffer control circuit, a write buffer, a vender testcircuit, and a Dout buffer control circuit.

The input (circuit) unit for receiving therein a signal inputted from anexternal terminal is regularly supplied with operating voltages throughthe power line VCC and the ground line VSS to ensure a response to theinput signal supplied from the external terminal. Further, the outputbuffer for forming an output signal is regularly supplied with theoperating voltages through the power line VCC and the ground line VSS tostably provide the output signal.

The input unit is provided, as an X system, with a RAS input buffer 1, aclock generator 2 for producing a RAS clock signal in response to asignal outputted from the RAS input buffer 1, an address input buffer 3supplied with an address signal, an X address latch 4 for taking in theaddress signal in response to the RAS clock signal R1B, and a CBRcounter 5 for producing an address signal used for a refresh operation.

The input unit includes, as a Y system, a CAS input buffer 18, a clockgenerator 19 for producing a CAS clock signal in response to a signaloutputted from the CAS input buffer 18, and a Y address latch 20 forcapturing a Y address signal inputted from the address input buffer 3 inresponse to the CAS clock signal. In addition to the components, theinput unit has an output enable input buffer 22, a write enable inputbuffer 23 and a data input buffer 24.

FIG. 3 is a circuit diagram showing one example of the X-system addressinput unit. An address signal IAYa is a refresh address signal formed bythe CBR counter 5 shown in FIG. 1. An address signal RAaB is an X-systemaddress signal supplied from the external terminal. These two addresssignals are supplied to their corresponding inputs of clocked invertercircuits CN1 and CN2. A refresh control signal IRF is brought to a highlevel upon refresh to thereby activate the clocked inverter circuit CN1and bring the clocked inverter circuit CN2 into an output highimpedance, whereby the refresh address signal IAYa is captured. When therefresh signal IRF is low in level, the clocked inverter circuit CN1 isbrought to an output high impedance and the clocked inverter circuit CN2is brought into an operation state, whereby the row-system addresssignal RAaB supplied from the external terminal is captured.

Outputs produced from the two clocked inverter circuits CN1 and CN2 arecommonly used and transmitted to a through latch via an invertercircuit. The through latch comprises an input clocked inverter circuitCN3, an inverter circuit IV3 and a feedback clocked inverter circuitCN4. A timing signal XAE0 is a row-system timing signal and is used toallow the through latch to perform a latch operation. Namely, when thetiming signal XAE0 is low in level, the input clocked inverter circuitCN3 is activated so that the feedback clocked inverter circuit CN4 isbrought to an output high impedance. Therefore, the address signal RAaBinputted from the external terminal or the refresh address signal IAYais captured through the input clocked inverter circuit CN3.

When the level of the timing signal XAE0 is changed from the low levelto the high level, the input clocked inverter circuit CN3 is brought toan output high impedance and alternatively, the feedback clockedinverter circuit CN4 is brought into an operating state. Therefore, asignal outputted from the inverter circuit IV3 is fed back to the inputside so that the captured address referred to above is latched in thethrough latch. Signals produced from the through latch are outputted ascomplementary internal address signals BXaB and BXaT through NOR gatecircuits and inverter circuits. The NOR gate circuits respectively opentheir gates during a period in which the timing signal XAE0 is high inlevel to thereby output the complementary internal address signals BXaBand BXaT corresponding to the latched address signal. When the timingsignal XAE0 is in a standby state of being brought to a low level incontrast with this, the internal address signals BXaB and BXaT are bothfixed to a high level and respective signals at the subsequent logicstages are respectively fixed to a predetermined signal level withoutreference to the previous memory access.

FIG. 4 is a circuit diagram showing one example of the predecoder whichreceives the internal address signals therein. Although not restrictedin particular, complementary internal address signals BX2Bi, BX2Tithrough BX4Bi and BX4Ti captured by the address input unit referred toabove are captured through NAND gate circuits whose gates are controlledbased on a testing control signal TASWTD. Eight predecode outputs AX20Bithrough AX27Bi are produced from NAND gate circuits by utilizing thecomplementary internal address signals BX2Bi, BX2Ti through BX4Bi andBX4Ti composed of three bits in combination.

The predecode outputs are respectively outputted through twocascade-connected inverter circuits used as output buffers. T in eachsignal referred to above indicates non-inversion (true) and B in eachsignal indicates inversion (bar). An explanation of the testing controlsignal TASWTD will be omitted because it has no immediate connectionwith the present invention. However, when the testing control signalTASWTD is rendered high in level, the gates of the NAND gate circuitsare closed so that the signals inputted to the respective NAND gatecircuits that constitute the predecoder, are all brought to a high levelwithout reference to the address signals BX2Bi, BX2Ti through BX4Bi andBX4Ti.

Since all the internal address signals BXaB and BXaT are fixed to thehigh level when the dynamic RAM is in the standby state, the signaloutputted from each NAND gate circuit in the input unit is rendered lowin level.

Since the input signal is rendered low in level, each NAND gate circuit,which constitute a decoder, fixes its output signal to a high level.Further, since a high level is supplied to the input of the precedingstage of each of two CMOS inverter circuits used for output, a signaloutputted from the preceding stage is rendered low in level and a signaloutputted from the subsequent stage thereof is rendered high in level.

Thus, the signal levels of the respective internal logic stages arefixed as described above in a non-operating state. Therefore, in orderto reduce the subthreshold leakage current described above, thefirst-stage NAND gate circuits are electrically connected to the groundline VSS in turn according to a signal transfer direction to produce thelow-level output signals, whereas the power sides thereof areelectrically connected to the sub power line VCTX. Since the P channelMOSFETs QP1 and QP2 and the like shown in FIG. 1, which are connected tothe power line VCC when being in the non-operating state, are turnedoff, the sub power line VCTX serves to reduce the subthreshold leakagecurrent that flows through the turned-off P channel MOSFETs constitutingeach of the NAND gate circuits referred to above.

Since the P channel MOSFETs and N channel MOSFETs of each CMOS circuit,which constitute each logic stage referred to above, are operated athigh speed, as will be described later, the threshold voltages thereofare lowered. On the other hand, the switch MOSFETs QP1, QP2, etc., whichconstitute the power switch referred to above, are set so that theirthreshold voltages are relatively increased, to thereby substantiallyprevent the flow of the subthreshold leakage current at the time thatthey are off.

Second-stage NAND gate circuits are respectively electrically connectedto the power line VCC to produce high-level output signals in responseto the low-level input signals supplied from the outputs of thefirst-stage NAND gate circuits. However, the ground sides thereof arerespectively electrically connected to the subground line VSTX. Sincethe N channel MOSFETs QN1, QN2 and the like shown in FIG. 1, which areelectrically connected to the ground line VSS when they are in thenon-operating state, are turned off, the subground line VSTX serves toreduce the subthreshold leakage current flowing through the turned-off Nchannel MOSFETs that constitute the NAND gate circuits. In a mannersimilar to the above, the switch MOSFETs QN1, QN2, etc., whichconstitute the power switch, are set so that their threshold voltagesbecome relatively large. As a result, the subthreshold leakage currentat the time that they are off, is substantially prevented from flowing.

Subsequently, each three-stage CMOS inverter circuit is activated by theVCTX and VSS lines in a manner similar to the first-stage NAND gatecircuits. Each fourth-stage CMOS inverter circuit is activated by theVCC and VSTX lines in a manner similar to the second-stage NAND gatecircuits. It is thus possible to reduce the subthreshold leakage currentthat flows when they are in a non-operating state.

FIG. 5 is a specific circuit diagram showing examples of the X decoder,a latch connected to the X decoder and a word driver. Although notrestricted in particular, symbols AX20 through AX27 respectivelyindicate signals produced by predecoding address signals A2 through A4composed of three bits with the above-described predecoder. Further,symbols AX50 through AX57 respectively indicate signals produced bypredecoding address signals A5 through A7 composed of three bits with apredecoder similar to the above predecoder. Each MOSFET Q3 whose gate issupplied with one of the predecoded signals AX20 through AX27 and eachMOSFET Q4 whose gate is supplied with one of the predecoded signals AX50through AX57 are electrically directly connected to one another so as toconstitute the X decoder, which is supplied with a select timing signalXDGB.

The X decoder is composed of a dynamic logic circuit. The X decoder isconstructed so that a P channel precharge MOSFET Q1 switch-controlled bya precharge signal XDP and the MOSFETs Q3 and Q4 constituting each logicblock are connected in series. Namely, a select/non-select decode signalis formed depending on whether or not a node precharged to a high levelby the precharge MOSFET Q1 is discharged through the MOSFETs Q2, Q3 andQ4 in response to a low level of the timing signal XDGB.

The latch comprises an inverter IV1 and the P channel MOSFET Q2 providedbetween the input of the inverter IV1 and a power terminal VCC andcontrolled by a signal XDGE outputted from the inverter IV1. The MOSFETQ2 forms a positive feedback circuit in response to the non-select levelcorresponding to the low level. The MOSFET Q2 prevents a non-select wordline from being selected by the inversion of the level at the above nodedue to the leakage current upon turning off of the MOSFETs Q3 and Q4.

Although not restricted in particular, the output signal XDGE of theinverter IV1 is of a select signal associated with four word lines WL0through WL3. Of these four word lines WL0 through WL3, one word line isselected which is specified or designated by four word line selecttiming signals XOMB through X3MB produced by decoding address signals A0and A1 of low-order bits and adding the select timing signal to them.

Namely, when the output signal XDGE of the latch is of a select levelcorresponding to a high level, a MOSFET Q5 is turned on. When the wordline select timing signal X3MB is changed from a high level to a lowlevel, a low-level input signal is supplied to a word driver composed ofa P channel MOSFET Q6 and an N channel MOSFET Q7 activated by a boosteror step-up voltage VCH and hence a word line WL3 connected to theiroutput terminals is raised from the low level to a high levelcorresponding to the step-up voltage VCH.

When the output signal XDGE of the latch is at the select levelcorresponding to the high level, other MOSFETs are also turned ontogether with the MOSFET Q5. However, since the word line select timingsignals X0MB through X2MB remain at the high level, the N channel MOSFETof the word driver is turned on so as to cause each of the word linesWL0 through WL2 to remain in a non-selected state indicative of a lowlevel. A P channel MOSFET Q8 is used as a latch MOSFET placed in thenon-select level. When the word line WL3 is at the non-select low level,the P channel MOSFET Q8 is turned on to thereby bring an input terminalof the word driver to the step-up voltage VCH so as to turn off the Pchannel MOSFET Q6. A P channel MOSFET Q9 is used as a precharge MOSFET.The P channel MOSFET Q9 is turned on in response to a low level of aprecharge signal WPH so that the input terminal of the word driver isprecharged to VCH.

When the output signal XDGE of the latch is of a non-select levelcorresponding to a low level, MOSFETs typified by the MOSFET Q5 are heldoff. Thus, even if either one of the word line select timing signalsX0MB through X3MB is changed from high to low levels, the P channelMOSFET Q8 is turned on in response to the low levels of the word linesWL0 through WL3 each associated with the precharge level without beingresponsive to its change. As a result, the latch that the high levelcorresponding to the VCH is fed back, is exerted on the input terminalof the word driver so that each of the word lines WL0 through WL3 or thelike is maintained at the non-selected state.

Since the amplitude of the signal inputted to the word driver composedof, for example, the MOSFETs Q6 through Q9 operated by the step-upvoltage VCH corresponding to the select level of each word line isincreased as in the circuits employed in the present embodiment, thethreshold voltage is relatively raised as in the power switches MOSFETsQP1, QN1, etc. Therefore, since the subthreshold leakage current at theturning off of the word driver can be substantially brought to naught,the MOSFETs are directly connected to the ground line VSS even in thesense of stabilization of the select/non-select level of each word line.However, since the inverter IV1 is reduced in signal amplitude in amanner similar to the predecoder and the input signal thereof may simplybe fixed to a high level as compared with the precharge signal XDP uponnon-selection of the word line so as to form or produce a low-leveloutput, the inverter IV1 may be connected to the ground line VSS and thesub power line VCTA.

A word driver, a latch MOSFET and a precharge MOSFET similar to thosereferred to above are connected even to a redundant word line RWL0. Theredundant word line RWL0 is selected in synchronism with the timingsignal XDGB and a redundant word line select signal XR0B produced by aredundant circuit composed of an unillustrated fuse circuit for storageof defective addresses and an address comparator for comparing eachdefective address and an input X address. Since, at this time, the AX20through A27 and AX50 through AX57 produced from the predecodercorresponding to a normal circuit or the word line select timing signalsX0MB through X3MB are brought to the non-select level, based on adefective-address comparison coincidence signal, no defective word lineis selected.

Although not restricted in particular, the memory array employed in thepresent embodiment is divided into a plurality of memory mats as will bedescribed later. Sense amplifiers SA, precharge circuits PC andinput/output lines are provided on both sides of each memory mat MAT.Although not restricted in particular, the sense amplifiers associatedwith odd-numbered complementary bit lines and even-numberedcomplementary bit lines are distributed to both sides to match thepitches of complementary bit lines disposed so as to intersect at rightangles to the word lines WL0 through WL3 or the like with the pitches ofthe sense amplifier and the precharge circuit. Owing to the placement ofsuch sense amplifiers SA, one sense amplifier can be placed at a pitchcorresponding to twice the pitch of each of the complementary bit lines.

Although not restricted in particular in the present embodiment, eachsense amplifier is set to a shared sense amplifier system. Signals SHLand SHR are of shared select signals. Although the left and right sidesare reversed at one view in the same drawing, the memory mat in the samedrawing is disposed on the left side as viewed from the right-side senseamplifier SA if the sense amplifier SA is considered as the center.Therefore, the select signal like SHL is supplied to the memory mat.Since the memory mat shown in the same drawing is disposed on the rightside as viewed from the left-side sense amplifier SA, the select signallike SHR is supplied to the memory mat.

FIG. 6 is a circuit diagram of one example of the mat control circuit.High-order address signals are decoded to produce or form mat selectsignals MS000, MS001 and MS002, for example. The memory mat MAT shown inFIG. 5 is selected by the mat select signal MS001. The mat select signalMS001 is supplied to four NAND gate circuits through two invertersconnected in tandem. The four NAND gate circuits are respectivelysupplied with timing signals x0 through x3 obtained by combining thedecode signals formed by decoding the address signals A0 and A1 with theword line select timing signals. Thus, the word line select timingsignals X0MB through X3MB are produced from the outputs of the NAND gatecircuits. This means that the predecode signals AX20 through AX27, AX50through AX57 and the timing signals x0 through x3 are commonly used forthe plurality of memory mats.

The precharge signals XDP and WPH and the operation timing signal XDGBof the row decoder are formed by combining the mat select signal MS001with X-system timing signals R1 and R2. Since the precharge signal WPHis used as a signal supplied to the gate of each P channel MOSFET whoseoperation is based on the step-up voltage VCH, the precharge signal WPHis converted into a level by a level converter circuit and the convertedlevel signal is outputted through an inverter activated by the step-upvoltage VCH. The mat select signals MS000 and MS002, each having theamplitude of a signal such as the source voltage VCC, are by-levelconverted into signal amplitude corresponding to the step-up voltage VCHby the level converter circuit, whereby the shared select signals SHRand SHL are formed.

Even in the case of the mat control circuit, since the input signals R1and R2 and MS001 through MS002 and the like are respectively fixed to alow level upon its non-operation as described above and are used to formhigh-level output signals, the mat control circuit is activated by thepower line VCC and the subground line VCTX in the same manner asdescribed above. Since second-stage CMOS inverter circuits are reverselysupplied with high-level input signals to form low-level output signals,the second-stage CMOS inverter circuits are electrically connected tothe sub power line VCTX and the ground line VSS. Subsequently,third-stage NAND gate circuits are activated by the power line VCC andthe subground line VCTX in the same manner as described above. Since thesignals x0 through x3 are fixed to a high level upon deactivation of themat control circuit, the third-stage NAND gate circuits are activatedcorrespondingly by the VCTX and VSS in accordance with theabove-described method. Further, the NAND gate circuits supplied withthe output signals thereof are activated by the VCC and VSTX.

FIG. 7 is a timing chart for explaining one example of the operation ofthe dynamic RAM shown in FIGS. 1 and 2. The row address strobe signalRASB is changed from the high to low levels to start the memory access.When the output signal R0B of the RAS input buffer 1 is changed from ahigh level to a low level, the RAS clock generator 2 changes the typicalrow-system timing signal R1B from a high to a low levels in response toits change. With the change of the timing signal R1B to the low level,the address signal Ai inputted from the address buffer 3 is captured orloaded into the X address latch 4 as an X address signal.

With the change of the timing signal R0B to the low level, the controlsignal φX of the power switch is changed from low to high levels and thecontrol signal φXB is changed from high to low levels. Thus, the supplyof the source voltage VCC to the sub power line VCTX is started by theturning on of the switch MOSFET QP1 and the supply of the groundpotential VSS to the subground line VSTX is started by the turning on ofthe switch MOSFET QN1. Namely, the supply of the voltages to the subpower line VCTX and the subground line VSTX is carried outsimultaneously and concurrently with the operations of the RAS clockgenerator 2 and the X address latch 4.

Thus, when an internal address signal X0 is generated in associationwith the latch operation of the X address latch 4, at least theinput-stage logic circuits of the respective circuits of the Xpredecoder 6, mat select circuit 7 and X address comparator 8 aresubstantially supplied with the source voltage VCC owing to the turningon of the switch MOSFET QP1. Further, the subground line VSTX issubstantially supplied with the ground potential VSS owing to theturning on of the switch MOSFET QN1. Thus, signals responsive to thesupplied voltages are formed without substantially interfering with theoperation. In association with the transfer of the signals from thelogic stages in the X predecoder 6 and the mat select circuit 7,potentials necessary for operation are successively supplied to the subpower line VCTX and the subground line VSTX by the switch MOSFETsstepwise-activated successively in their signal transfer directions, sothat predecode signals X1 and X2 and a mat select signal X3 are formed.

The control signals φA and φAB are respectively changed to a high leveland a low level with lags behind the control signals φX and φXB tothereby start the supply of the source voltage VCC to the sub power lineVCTA for the array block and the supply of the ground potential VSS tothe subground line VSTA for the array block. Since the timing providedto output the predecode signal X1 formed in the above-described X-systemcircuit and an output signal X4 of the mat control circuit 9 makes alead time, the sub power line VCTA and the subground line VSTA used forthe array block are respectively set to desired potentials.

Thus, in the array block, one word line WORD is raised to the high levelfrom the low level in response to a select signal X5 produced from the Xdecoder 12. Thereafter, the common source switch 16 is turned on inresponse to the change of a sense amplifier activation signal S0 to alow level to thereby respectively vary common sources SP and SN of eachsense amplifier to a high level and a low level. As a result, amicrosignal is read into its corresponding complementary bit lines ofthe memory array and thereafter the operation for amplification of themicrosignal is started.

A column address strobe signal CASB is changed from a high level to alow level to capture a Y-system address signal. Namely, when the outputsignal of the CAS input buffer 18 is changed from a high level to a lowlevel, the CAS clock generator 19 generates an address take-in orcapturing timing signal in response to its change to thereby allow the Yaddress latch 20 to capture the address signal Ai inputted from theaddress input buffer 3 therein as a Y address signal.

The address signal Y1 taken in the Y address latch 20 is supplied to theY predecoder 28 and the Y address comparator 29. An address signal Y0 issupplied to the address variation detector 25 from which an addressvariation detection signal C0 is generated. In response to the signalC0, the equalizing pulse generator 26 generates an equalize pulse C1 toequalize the input signal supplied to the input terminal of the mainamplifier 30. In response to the signal C0 and the timing signaloutputted from the CAS clock generator 19, the main amplifier controlcircuit 27 generates a main amplifier control signal C2.

Since the Y decoder 33 generates a Y select signal in response to apredecode signal Y2, a read signal D0 is transferred to the inputterminal of the main amplifier 30 and a signal D1 obtained by amplifyingthe read signal D0 with the main amplifier 30 is transferred to theoutput buffer 37 through a data selector 35 as an input signal D2therefor. The output buffer 37 is activated by a timing signal C3outputted from the Dout buffer control circuit 36 to transmit outputdata DATA therefrom.

In a write mode, a write clock generator 21 judges or determines that asignal outputted from the write enable input buffer 23 is low in level.As a result, the write buffer 32 is activated so that data inputted fromthe data input buffer 24 is transferred to the complementary bit linesof the memory array, which are selected by the Y select signal.

The control signals φY and φYB associated with the Y-system and writecircuit are respectively slowly varied to a high level and a low levelin suitable timing in the course of the select operation of the X-systemreferred to above. Therefore, the switch MOSFETs QP5 and QN5respectively connected to the sub power line VCTY and the subground lineVSTY associated with the Y-system and write circuit are set so as tohave desired current supply abilities with timing provided to start theoperations of the respective circuits of the Y-system while currentsoutputted from the switch MOSFETs QP5 and QN5 are being increased slowlyaccording to gentle changes in their gate voltages.

The sub power lines VCTX, VCTA and VCTY and subground lines VSTX, VSTAand VSTY, which are respectively divided into three as described above,are respectively substantially brought into a floating state uponreaching their non-operating states. In this condition, the controlsignals φX and φXB, φA and φAB, and φY and φYB are respectivelysuccessively generated with delays upon the memory access while thesubthreshold leakage current produced in each logic circuit is beingsuppressed. Since the drive current required to vary the voltage appliedto the gate of each of the switches MOSFETs respectively controlledbased on the control signals φX and φXB, φA and φAB, and φY and φYB, andthe supply current required to vary the voltages at the sub power linesVCTX, VCTA and VCTY and the subground lines VSTX, VSTA and VSTY to adesired voltage slowly increase in terms of time, the occurrence of thepeak current is suppressed and the influence of the currents on theoperating speeds of the respective circuit blocks can be substantiallyavoided.

FIGS. 8 and 9 are respectively block diagrams showing one embodiment ofthe dynamic RAM to which the present invention is applied. FIG. 8 showsa memory array and its peripheral selection circuits. FIG. 9 illustratesan input/output interface such as an address buffer, an input/outputbuffer or the like, and a timing control circuit.

Referring to FIG. 8, a sense amplifier SA01 is provided so as to beinterposed between two memory mats MMAT0 and MMAT1. Namely, the senseamplifier SA01 serves as a shared sense amplifier used selectively forthe two memory mats MMAT0 and MMAT1. An input/output portion of thesense amplifier SA01 is provided with an unillustrated select switch andis electrically connected to complementary bit lines (or also called"complementary data lines or complementary digit lines") of the memorymat MMAT0 or MMAT1.

Other memory mats MMAT2 and MMAT3, MMAT4 and MMAT5 and MMAT6 and MMAT7are respectively provided in pairs and provided with sense amplifiersSA23, SA45 and SA67 in common. A single memory array MARY0 is composedof a total of eight memory mats MMAT0 through MMAT7 and the four senseamplifiers SA01 through SA67 referred to above. A Y decoder YDEC isprovided for the memory array MARY0. A memory array MARY1 is providedsymmetrically with respect to the memory array MARY0 with the Y decoderYDEC interposed therebetween. Although an internal configuration of thememory array MARY1 is omitted, the memory array MARY1 is similar inconfiguration to the memory array MARY0.

Decoders XD0 through XD7 are provided with respect to the memory matsMMAT0 through MMAT7 respectively. These decoders XD0 through XD7respectively decode a signal AXi outputted from a predecoder XPD toproduce or form four word line select signals. Word drivers WD0 throughWD7 for producing or forming word line select signals, based on signalsoutputted from the decoders XD0 through XD7 and signals outputted frommat control circuits MATCTRL01 through MATCTRL67 to be next describedare provided. Word drivers corresponding to spare or reserve word linesfor defective relief are also included in the word drivers.

The mat control circuit MATCTRL01 is provided with respect to the pairof memory mats MMAT0 and MMAT1. Similarly, the mat control circuitsMATCTRL23, MATCTRL45 and MATCTRL67 are respectively provided even withrespect to other pairs of memory mats MMAT2 and MMAT3, MMAT4 and MMAT5,and MMAT6 and MMAT7. The mat control circuits MATCTRL01 throughMATCTRL67 are activated in response to a mat select signal MSi, a signalXE, a sense operation timing signal φSA and signals obtained by decodingaddress signals of the two rightmost bits. Of these, one mat controlcircuit relative to the selected memory mat outputs a signal XiB or thelike for selecting one of the four word lines.

Otherwise, each of the mat control circuits MATCTRL01 through MATCTRL67outputs a select signal for holding ON a bit line select switchcorresponding to either of the left and right memory mats in associationwith the selected memory mat referred to above and for turning OFF bitline select switches associated with the non-selected memory mats, and atiming signal for starting the amplifying operation of each senseamplifier. Further, each of the mat control circuits has the function ofcontrolling either one or both of each sense amplifier and each bit lineselect switch upon standby at a refresh operation to be described laterto thereby bring each bit line into the floating state.

Since the output of the select signal XiB or the like is prohibitedbased on a low level of the signal XE when an access to a defective wordline is performed, the operation for selecting the defective word lineis stopped. On the other hand, since a select signal XRiB on theredundant circuit side is formed, the reserve word line is placed in aselected state.

Referring to FIG. 9, a timing control circuit TG determines an operationmode in response to a row address strobe signal /RAS, a column addressstrobe signal /CAS, a write enable signal /WE and an output enablesignal /OE supplied from an external terminal, and correspondingly formsor produces various timing signals necessary for the operation of eachinternal circuit. In the same drawing, symbol/means that a low level isan active level.

Signals R1 and R3 are of row-system internal timing signals and are usedfor a row-system select operation to be described later. The timingsignal φXL is used as a signal for capturing a row-system address andholding it and is supplied to a row address buffer RAB. Namely, the rowaddress buffer RAB captures addresses inputted from address terminals A0through Ai in response to the timing signal φXL and causes a latch tostore the same therein.

The timing signal φYL is used as a signal for capturing column-systemaddresses and for causing a latch to hold the same therein and issupplied to a column address buffer CAB. Namely, the column addressbuffer RAB takes in the addresses inputted from the address terminals A0through Ai in response to the timing signal φYL and causes the latch tostore them therein.

The signal φREF is a signal generated upon a refresh mode and issupplied to a multiplexer AMX connected to an input portion of the rowaddress buffer. The multiplexer AMX is controlled based on the signalφREF so as to select a refresh address signal formed by a refreshaddress counter RFC. The refresh address counter RFC counts a refreshstepping pulse φRC produced by the timing control circuit TG to therebygenerate the refresh address signal. The present embodiment isconstructed so as to have auto-refresh and self-refresh capability to bedescribed later.

The timing signal φX is of a row or word line select timing signal andis supplied to a decoder XIB from which four word line select timingsignals XiB are formed or produced based on the signals obtained bydecoding the address signals of the two rightmost bits. The timingsignal φY is of a column select timing signal and is supplied to acolumn-system predecoder YPD from which column select signals AYix, AYjxand AYkx are outputted.

The timing signal φW is a control signal for providing instructions fora write operation and the timing signal φR is a control signal forproviding instructions for a read operation. The timing signals φW andφR are supplied to an input/output circuit I/O. Upon the writeoperation, an input buffer included in the input/output circuit I/O isactivated so that the output buffer is brought to an output highimpedance. Upon the read operation on the other hand, the output bufferis activated to bring the input buffer to an output high impedance.

The timing signal φMS is a signal for providing instructions for a matselect operation and is supplied to the row address buffer RAB. A matselect signal MSi is outputted in synchronism with this timing. Thetiming signal φSA is used as a signal for providing instructions for theoperation of each sense amplifier. Based on the timing signal φSA, thetiming signal φMS is also used to form control signals for a prechargeend operation of complementary bit lines and a separate operation of bitlines on the non-selected memory mat side as well as to form anactivation pulse for each sense amplifier.

In the present embodiment, a row-system redundant circuit X-RED istypically shown by way of illustrative example. Namely, the abovecircuit X-RED includes a memory circuit for storing each defectiveaddress therein and an address comparator. The stored defective addressis compared with an internal address signal BXi outputted from the rowaddress buffer RAB. If they do not coincide with each other, then thesignal XE is brought to a high level and a signal XEB is brought to alow level to thereby make the operation of the normal circuit valid oreffective. When the input internal address signal BXi coincides with thestored defective address, the signal XE is rendered low in level toprohibit the select operation of each defective word line in the normalcircuit. Further, the signal XEB is rendered high in level to output theselect signal XRiB for selecting one reserve word line.

Although omitted in FIG. 9, a circuit similar to the row-system circuitis provided even within a column system. When a memory access to adefective bit line is detected by the circuit, the operation for theselection of the defective bit line by a column decoder YD is stoppedand instead a signal for selecting each of the bit lines provided asspares is formed.

FIG. 10 is a fragmentary circuit diagram showing one example of thememory array employed in the dynamic RAM according to the presentinvention. In the same drawing, four word lines and two pairs ofcomplementary bit lines in the memory mat MMAT0, a sense amplifierrelated to these, a precharge circuit, etc. are typically illustrated byway of example. The memory mat MMAT1 is shown as a black box. Circuitsymbols are typically added to MOSFETs that constitute individualcircuits associated with a pair of complementary bit lines BLL and /BLL.

A dynamic memory cell comprises address selection MOSFETs Qm andinformation storage capacitors Cs. The gate of one address selectionMOSFET Qm is electrically connected to a word line WLi. The drain of theMOSFET Qm is electrically connected to the bit line /BLL and theinformation storage capacitor Cs is electrically connected to the sourcethereof. Other electrodes of the information storage capacitors Cs arecommonly used and are supplied with a plate voltage VPL.

The pair of bit lines BLL and /BLL is disposed in parallel as shown inthe same drawing and suitably intersect as needed to strike a capacitybalance between the bit lines, for example. The complementary bit linesBLL and /BLL are electrically connected to an input/output node of eachsense amplifier by switches MOSFETs Q1 and Q2. The sense amplifier iscomposed of N channel MOSFETs Q5 and Q6 whose gates and drains arecross-connected and provided in latch form and P channel MOSFETs Q7 andQ8 whose gates and drains are cross-connected and provided in latchform. The sources of the N channel MOSFETs Q5 and Q6 are electricallyconnected to a common source line CSN. The sources of the P channelMOSFETs Q7 and Q8 are electrically connected to a common source lineCSP. A power switch MOSFET Q14 for the P channel MOSFETs is connected tothe common source line CSP as shown by way of illustrative example. Whena timing signal φSAP is rendered low in level, the MOSFET Q14 is turnedon so as to supply a voltage required to activate each sense amplifier.An unillustrated N channel MOSFET is connected to the common source lineCSN associated with the N channel MOSFETs Q5 and Q6 so as to supply acircuit ground potential for providing line operation timing.

A precharge circuit composed of a MOSFET Q11 for short-circuiting thecomplementary bit lines and switches MOSFETs Q9 and Q10 for supplying ahalf precharge voltage HVC to the corresponding complementary bit linesis connected to the input/output node of each sense amplifier. The gatesof the MOSFETs Q9 through Q11 are commonly supplied with a prechargesignal PCB. MOSFETs Q12 and Q13 form a column switch switch-controlledin response to a column select signal YS. In the present embodiment,four pairs of bit lines can be selected by one column select signal YS.Therefore, the column select signal YS is supplied common to the gatesof switch MOSFETs constituting column switches connected to input/outputnodes of four sense amplifiers associated with the two pairs of bitlines illustratively shown in the same drawing and the remainingunillustrated two pairs of bit lines. The four pairs of bit lines andthe four pairs of input/output lines I/O are respectively connected toone another through the switch MOSFETs.

FIG. 11 is a cross-sectional view showing, as one embodiment, a devicestructure for describing the dynamic RAM according to the presentinvention. In the present embodiment, a device structure for the memoryarray and the peripheral portion referred to above is shown as a typicalone by way of illustrative example. In regard to the storage capacitorof each memory cell, a polysilicon layer SG corresponding to a secondlayer is used as a storage node and is electrically connected to thesource and drain of one the address selection MOSFETs. The polysiliconlayer is shaped as a fin structure and is formed by a plate electrodecomposed of a polysilicon layer TG corresponding to a third layer with athin gate insulating film interposed between the two layers. The gate ofeach address selection MOSFET is composed of a polysilicon layer FGcorresponding to a first layer. The source and drain of each addressselection MOSFET are electrically connected to a metal interconnectionor wiring layer M1 corresponding to a first layer, such as aluminum orthe like with the FG, SG and TG interposed therebetween. Each bit lineis formed of the interconnection layer M1.

The peripheral portion is formed with two N channel MOSFETs. The metalwiring layer M1 is electrically connected to the source and drain ofeach MOSFET by a contact LCNT. Alternatively, the metal wiring layer M1is electrically connected to the polysilicon layer FG by a contact FCNT.The above-described metal wiring layer M1 and wiring layer M2 areelectrically connected to each other through a first through-hole TH1,and the wiring layer M2 and a wiring layer M3 corresponding to a thirdlayer are electrically connected to each other through a secondthrough-hole TH2. When an input signal is supplied to the gate electrodeof the MOSFET through the wiring layer M2, the input signal is taken inthe wiring layer M1 used as a dummy through the first through-hole TH1as described above and hence the input signal is introduced into thepolysilicon layer FG used as the gate electrode through the wiring layerM1 and the contact LCNT.

The wiring layer M3 for supplying the input signal is electricallyconnected to the wiring layer M2 through the second through-hole TH2.When an output signal is supplied to the next-stage circuit, forexample, the wiring layer M1 is electrically connected to the wiringlayer M2 used as a dummy through the first through-hole TH1. Thus, theoutput signal is introduced into the wiring layer M3 through the secondthrough-hole TH2 with the wiring layer M2 interposed between the wiringlayers M1 and M3.

Since the complementary bit lines are half-precharged in thenon-operating state, a half precharge voltage is applied between thegate and source of each MOSFET as a reverse bias. Therefore, nosubthreshold leakage current occurs in each address selection MOSFET. Itis however considered that when each word line becomes a non-selectionlow level and the complementary bit lines are brought to VSS by theamplifying operation of each sense amplifier, the information chargestored at the high level is lost or disappears due to the subthresholdleakage current.

If the subthreshold leakage current at such an address selection MOSFETturns into a problem, then the channel length is made long and thethreshold voltage is increased. Alternatively, well regions, in whichmemory cells each composed of the address selection MOSFETs are formed,are separated from each other and a negative substrate back bias voltageis supplied to each well region to increase the effective thresholdvoltage. In order to separate only the well regions for forming thememory cells from each other and supply the substrate back bias voltagethereto, a semiconductor substrate is shaped in the known triple wellstructure. Namely, each of the N channel MOSFETs, which constitute theperipheral circuits such as the decoder, etc., needs to have the lowthreshold voltage as described above in order to increase the operatingspeed thereof. Each well region, in which the N channel MOSFET isformed, is biased to the circuit ground potential VSS.

When a large circuit block is provided with a set of a sub power lineand a subground line alone, it is necessary to provide switch MOSFETswhose gate widths are large and to reduce the on resistance of eachswitch MOSFET so as to provide the supply of current to the circuitblock. This is because when a voltage drop occurs in the sub power line,the threshold voltage of each of P channel MOSFETs constituting the samecircuit block increases equivalently and the lowering of speedup by theuse of each MOSFET whose threshold voltage is low is canceled due to itsincrease. Similarly, this will happen even in the case of therelationship between the threshold voltage of each N channel MOSFET andthe on resistance of each MOSFET connected to the subground line.

It is thus necessary to reduce the voltage drop in the order of severaltens of mV at maximum with respect to the average operating current ofthe same circuit block with a view toward keeping the effect of speedingup its operation. In the dynamic RAM according to the previousembodiment, for example, switch MOSFETs each having gate widths rangingfrom 5000 μm to 20000 μm are needed. It is thus necessary to charge ordischarge the capacitor of the gate which is large in width when eachswitch MOSFET is turned on.

It is desirable that the sub power line and the subground line are usedin a number of circuit blocks wherever practicable to reduce currentconsumption at the time of the non-operation as the entire circuit. Tocope with it, it is however necessary to turn on the above-describedpower switch MOSFET with a timing provided as quickly as possible afterthe inputting of RASB. For example, the sum of the gate widths of theMOSFETs connected to the power line VCC and the ground line VSS at theinput unit becomes about 10000 μm, whereas the sum of the gate widths ofthe MOSFETs connected to the sub power line and the subground line atthe X predecoder and the mat select circuit reaches about 15000 μm.Thus, whether or not the sub power line and the subground line should beconnected to the X predecoder and the mat select circuit, depends on thechange of the leakage current at the non-operation (deactivation) to thehalf the leakage current. It is thus important that each switch MOSFETis turned on before the X predecoder and the mat select circuit arestarted (before and after 5n seconds after the inputting of RASB, forexample).

Since it is necessary to charge or discharge the gate capacitor referredto above in a short time, the sub power line and the subground line areprovided in a pair as the entire circuit and the P channel switch MOSFETand the N channel switch MOSFET are respectively provided singly. Indoing so however, a large peak current, which falls within a range of0.5 A to 1.0 A, will flow when such switches MOSFETs are turned on. Whensuch a large peak current is superimposed on the operating currentflowing in each internal circuit, a large problem arises in terms oflong-term reliability due to disconnection or the like caused by noiseor a concentrated current.

As described above, in accordance with the present embodiment, theconcentration of current on each switch MOSFET at the time of its switchcontrol is dispersed on a time basis by dividing the sub power lines andthe subground lines into three portions as a whole as described aboveand by establishing a difference between the timing provided to starteach MOSFET and the timing provided to operate each of the switchesMOSFETs divided in plural form, so as to stepwise activate each switchMOSFET. However, the peak current can be also controlled by simplycollectively utilizing the sub power lines and the subground lines incommon in the form of several blocks, providing the switch MOSFETs inplural form and setting a difference between the start timings. In thiscase, the layout of the sub power lines and the subground lines becomeseasy as compared with the case in which the sub power lines and thesubground lines are divided into pieces between the blocks. Further,since the sub power lines and the subground lines also increase inparasitic capacity, an advantage is obtained that variations in thevoltages respectively applied to the sub power lines and the subgroundlines are reduced due to the occurrence of an instantaneous largecurrent.

FIG. 12 is a block diagram for describing one embodiment of the presentinvention. A power line VCC and a sub power line VCT, switch MOSFETsassociated therewith, inverters, which constitute delay circuits forforming signals for controlling the MOSFETs, and circuit blocks suppliedwith operating voltages from the inverters, are shown in the samedrawing. Subground lines and a ground line for the respective circuitblocks employed in the present embodiment and switch MOSFETs associatedtherewith are omitted because they are similar to those on the sourcevoltage VCC side.

In the present embodiment, the switch MOSFETs such as MOSFETs QP1through QP4 for connecting the sub power line VCT and the power line VCCto one another are respectively provided so as to correspond to circuitblocks 1 through 4. The sum of gate widths of the individual switchMOSFETs QP1 through QP4 is set to such a value as to fall within a rangein which the allowable voltage of the sub power line VCT varies due tothe on resistance of each switch MOSFET referred to above. A controlsignal φ supplied to the gates of the switch MOSFETs QP1 through QP4 istransferred to each of inverters IV1 through IV7 as a signal delayed inturn by each of inverters IV1 through IV7 in order of transferring it toeach of the circuit blocks 1 through 4 in turn.

Thus, when the sub power line VCT is shared between the plurality ofcircuit blocks 1 through 4, the X-system circuit, the array block andthe Y-system and write circuit are respectively associated with thecircuit blocks in the above-described dynamic RAM, for example. In thepresent embodiment, the circuit block 1 is supplied with an operatingvoltage from the switch MOSFET QP1 so as to perform a logic operation inresponse to an input signal IN. At this time, the voltage VCC suppliedfrom the MOSFET QP1 is not sufficiently delivered or transferred to thecircuit blocks far away from the input signal side as in the case ofother circuit blocks 2 through 4 due to the distributed resistance ofthe sub power line VCT. However, since these circuit blocks performsignificant circuit operations in response to signals outputted from thepreceding-stage circuit blocks, a substantial problem does not occur.Namely, when a significant output signal corresponding to the inputsignal IN is transferred to the next-stage circuit block 2, the switchMOSFET QP2 is turned on so that the voltage VCC for performing a logicoperation associated with it is supplied to the circuit block 2. Thus,since the delay in signal at each logic stage and the supply of thevoltage to the sub power line VCT are carried out substantially insynchronism with each other, a substantial delay in operating speed doesnot occur.

The signal delay time developed in each logic circuit does notnecessarily coincide with the operation of each switch MOSFET referredto above. This is due to the fact that, since a delay in supplying thepower delays the operation for outputting a high level in response toits delay, a substantial logic output is actually formed dependent onthe voltage of the sub power line VCT, which is supplied from eachswitch MOSFET. Thus, since the operating speed of each logic circuitbecomes slow when the control on each switch MOSFET is extremelydelayed, the difference in time between the respective switch MOSFETs isset so that the peak current referred to above falls below the allowancevalue and the voltage is stepwise supplied to the power line.

FIG. 13 is a block diagram for explaining another embodiment of thepresent invention. The same drawing shows an example in which a subpower line and a subground line are respectively divided into pluralforms for each of the circuit blocks. In the present example, the gatewidth of each switch MOSFET, which is determined by each of the valuesof allowable voltages of each sub power line and each subground line,which varies due to an on resistance of each switch MOSFET, can bereduced as compared with the case in which the sub power line is sharedbetween the circuit blocks.

As a result, charge and discharge currents, which flow through the gatesof the respective switch MOSFETs QP10 through QP40 or the like, arereduced. Further, a peak current can be reduced by starting the switchMOSFETs in turn with the passage of time, using the pair of sub powerline and subground line for each circuit block activated substantiallyin the same timing. At the same time, the startup of each switch MOSFETcan be speeded up because the gate width of each switch MOSFET is smallas compared with the case in which the sub power line is not dividedinto plural forms. When circuit blocks free of execution of theircircuit operations exist as in the Y-system and the write circuit at therefresh operation in the dynamic RAM, their corresponding switch MOSFETscan remain in an off state and hence circuit's current consumption canbe reduced.

FIG. 14 is a circuit diagram showing one example of an X-system inputportion or unit employed in the dynamic RAM according to the presentinvention. A switch MOS control unit or circuit which has been omittedin the above-described embodiment and an X-system input portionassociated with it are shown in the same drawing in combination.

The switch MOS control unit forms or produces a signal SWC for startinga switch MOSFET in response to a clock signal generated at the earliesttiming in response to an input signal RASB. Thus, a first stage of a RASclock generator, an X address buffer and the switch MOS control unit,activated before each switch MOSFET is turned on, is not connected tothe sub power line and the subground line. An X predecoder and thesubsequent stage of the RAS clock generator are electrically connectedto the sub power line VCT and the subground line VST. Gates andinverters at which output signals at the time of deactivation are low(L) in level, are electrically connected to the sub power line VCT.Gates and inverters, at which output signals are high (H) in level, areelectrically connected to the subground line VST as described above.Thus, each switch MOSFET is turned off upon deactivation to reduce asubthreshold leakage current developed in such gates and inverters,whereby a current consumed during standby is reduced.

Since the switch MOSFETs are parallel-connected in plural form and theirgates are supplied with delayed signals, the switch MOSFETs aresuccessively turned in domino or stepwise fashion. While suppressingtheir driving and a peak current produced due to their turning-on, thesub power line VCT and the subground line VST are supplied with theircorresponding VCC and VSS voltages. A signal SET inputted to the switchMOS control circuit is an initialization signal, which is used togenerate the switch MOS start signal SWC upon turning on of the power inthe circuit and turn on each switch MOSFET so as to increase the voltageon the sub power line VCT. A signal TEST is of a test signal, which isused to generate a start signal from the outside for thereby forciblyturning on each switch MOSFET. The signal TEST is pulled down to groundthrough the resistance of its input node and is normally fixed to a lowlevel.

Even if the RASB is brought to the high level, each switch MOSFET isturned off by a signal φτ produced by delaying a RAS reset signal by atime τ (˜5n seconds) so as to avoid the immediate turning off of theswitch MOSFET. This is due to the fact that, since the circuit isprecharged after the RASB has been brought to the high level, eachswitch MOSFET is held on during that time.

When the dynamic RAM enters into a self-refresh mode (CBR refresh), aself-refresh signal SELF is generated with timing A in response to theinput of CBR (CAS before RAS) as shown in a timing chart in FIG. 15. Inorder to turn off the switch MOSFET, except for the case where thedynamic RAM is actually performing a refresh operation in theself-refresh mode, to thereby reduce the subthreshold leakage current,the switch MOSFET can be controlled even by an internal signal IRASB.

If the refresh operation is set to a concentrated refresh forconcentratedly performing refresh on all the memory cells and thereafterbringing them into a deactivated state until the next refresh, asopposed to setting the operation to distributed refresh for uniformlydispersing and performing one cyclic operation required to refresh allthe memory cells once, within its holding time, then the number of timesthat the switch MOSFET is controlled can be reduced. A multiplexer MPXprovided within the X address buffer selects an address signal ADi or arefresh address signal RADi inputted from the external terminal inassociation with the refresh control signal SELF and takes it therein.

The SET signal is used to generate the start signal SWC for each switchMOSFET upon turning on the power in the circuit and turn on the switchMOSFET so as to raise the voltage applied to the sub power line VCT. Asan alternative to the signal SET, this processing may be performed by aMOSFET diode-connected between the power line VCC and the sub power lineVCT. In this case, it is unnecessary to turn on the switch MOSFET uponpower-up. If each of nodes in internal circuits at power-up is set to apotential at deactivation by the SET signal, then the supply of currentto each of the internal circuits at power-up is all made by the powerline VCC but not performed by the sub power line VCT. Therefore, thevoltage of the sub power line VCT can be raised even by a diode having alow current supply ability.

Since the internal circuit is placed in an activated state when eachswitch MOSFET is in an on state upon power-up, the subthreshold leakagecurrent flows. Since the potential on the sub power line VCT does notincrease to VCC when the above-described diode is used, it is possibleto prevent the subthreshold leakage current from occurring. A furthereffect can be obtained when MOSFETs, supplied with a substrate bias byusing a substrate back bias voltage generator, are used. In the MOSFETsto which the substrate bias is applied, the threshold voltage of eachMOSFET becomes low and hence a large subthreshold leakage current flowssince the substrate back bias voltage generator does not generate asufficient substrate bias voltage upon power-up.

Circuits such as the above-described address buffer, etc., which aredisconnected from the sub power line and the subground line, and theabove-described switch MOSFETs utilize high threshold-voltage typeMOSFETs to reduce the subthreshold leakage current at the time that theswitch MOSFETs are brought into the off state. In the present invention,a method of forming MOSFETs, whose channel length is made long using thedependence of the threshold voltage of each MOSFET on its gate length,is used as a method of forming the MOSFETs whose threshold voltage ishigh. Counter-doping to be described later is used to realize thedependence of a desired threshold voltage on the gate length.

By realizing two or more types of threshold voltages using thegate-length-dependence of the threshold voltage of each MOSFET, at leasttwo masks (for P and N channels) can be reduced and the number ofmanufacturing process steps can be reduced as compared with a method ofrealizing two or more kinds of threshold voltages by making anion-implantation division using conventional photomasks.

FIGS. 16A and 16B are respectively schematic structural sectional viewsshowing examples of MOSFETs employed in a semiconductor integratedcircuit device according to the present invention. FIG. 16A shows anormally-used MOSFET and FIG. 16B illustrates a counter-doped MOSFET.The term counter-dope refers to a means for realizing a MOSFET, which isdifferent from the normal MOSFET shown in FIG. 16A, in that it isexcellent in short channel characteristic and has a low thresholdvoltage, by introducing the same conductive impurities as thosecontained in the source and drain of a substrate surface into the MOSFETin small concentrations.

FIG. 17 is a characteristic diagram illustrating the relationshipbetween a gate length of an N channel MOSFET and its threshold voltage.In the same drawing, symbol ∘ indicates typical values of the MOSFETshown in FIG. 16A having the conventional structure. Symbol  indicatestypical values of a counter-doped transistor. These values becomevariations lying between upper and lower broken lines and between upperand lower solid lines, for example, due to process variations.

The allowable minimum value of each of the threshold voltages of theMOSFETs whose gate lengths are short, which constitute each internalcircuit connected to the sub power line and the subground line asdescribed above, is determined according to the subthreshold leakagecurrent of each internal circuit at the time that each switch MOSFET isheld on. In the example of the dynamic RAM shown in FIGS. 1 and 2, theallowable minimum value is about 0 V at room temperature because the sumof the gate widths of the MOSFETs is about 700,000 μm. When the worstvalue of the threshold voltage, which is attributed to the processvariations, is set to 0 V, the gate length of the MOSFET becomes 0.45 μmand the threshold voltage thereof becomes 0.29 V (both are typicalvalues), for example, when the MOSFET having the conventional structureis used. On the other hand, when the counter-doped MOSFET is used, thegate length thereof becomes 0.45 μm and the threshold voltage thereofbecomes 0.2 V (both are typical values).

At this time, the threshold voltage causes variations within B indicatedby a thick-line frame in FIG. 17 due to the process variations in thecase of the MOSFET having the conventional structure. When thecounter-doped MOSFET is used, the threshold voltage thereof causesvariations within A indicated by a thick-line frame in FIG. 17 due tothe process variations. Since the variations in threshold voltage due tothe variations in gate length are reduced as a result of the suppressionor control of a short channel effect by the counter-dope process, thetypical threshold voltages can be reduced, so that a logic circuit orthe like can be designed using higher-speed MOSFETs.

The minimum value of each of the threshold voltages of the MOSFETsconstituting the circuits disconnected from the sub power line VCT andthe subground line VST are also determined depending on theirsubthreshold leakage currents. In the dynamic RAM shown in FIGS. 1 and2, the minimum value becomes about 0.2 V at room temperature. Thus, whena MOSFET having the conventional structure is used in the same manner asdescribed above, the gate length thereof becomes 0.53 μm and thethreshold voltage becomes 0.42 V (both are typical values), whereas whenthe counter-doped MOSFET is used, the gate length thereof becomes 0.55μm and the threshold voltage becomes 0.30 V (both are typical values).The above-described threshold voltages respectively cause variationswithin D and C indicated by thick-line frames in FIG. 17 due to theprocess variations. Even in the case of the MOSFETs referred to above,high-speed MOSFETs, which are low in threshold voltage due to thecounter dope, can be utilized.

When the threshold voltages of the MOSFETs for providing connectionsbetween the sub power line and the power line and between the subgroundline and the ground line vary, subthreshold leakage currents at theirturning off greatly vary. Thus, MOSFETs, in which variations in theirthreshold voltages due to the process variations are as small aspossible and their gate lengths are long, are used as these switchMOSFETs. To cope with this, the gate length at which a curve indicativeof the dependence of the threshold voltage on the gate length issubstantially flat, may be set to a range from 0.7 μm to 0.8 μm or morein FIG. 17.

When the gate length of the switch MOSFET is made long, its onresistance is reduced. It is therefore necessary to increase the gatewidth thereof. Thus, it must be noted that the peak current at turningon of the switch MOSFET also increases. Namely, if the gate-lengthdependence of the threshold voltage is low, then the gate length of theswitch MOSFET may be set so as to become as short as possible. Thus, inthe present embodiment, when the MOSFET having the conventionalstructure is used, the gate length thereof becomes 0.8 μm and thethreshold voltage thereof becomes 0.5 V (both are typical values).Further, when the counter-doped MOSFET is used, the gate length thereofbecomes 0.7 μm and the threshold voltage thereof becomes 0.35 V (bothare typical values). The threshold voltages respectively vary within Fand E ranges indicated by thick-line frames in FIG. 17.

Although the MOSFET of the conventional structure rather than thecounter-doped MOSFET bring about an advantageous effect because of itshigh threshold voltage from the viewpoint of the reduction in leakagecurrent, the leakage current of the switch MOSFET is very small ascompared with that of each circuit disconnected from the sub power lineand the subground line referred to above. Therefore, this effect can beneglected. Since a MOSFET having a short channel, a low thresholdvoltage and high drive ability can be used for the switch MOSFET as aresult of the control on the short channel effect by the counter doping,the gate width of the present switch MOSFET can be made smaller thanthat of the MOSFET having the conventional structure and the peakcurrent can be reduced.

FIG. 18 is a characteristic diagram for describing the presentinvention. In the same drawing, the vertical axis indicates a peakcurrent and an increase in RAS access time tARAS and the horizontal axisindicates a time difference per step of a control signal for each switchMOSFET. The result of a computer simulation using an actual dynamic RAMis illustrated in the drawing. A position or point where the timedifference per step of the control signal for each switch MOSFET is 0,indicates that all the switch MOSFETs are simultaneously turned on.

The switch MOSFETs are divided into five. The gate width of each Pchannel MOSFET is 3000 μm and the gate width of each N channel MOSFET is900 μm as shown in the drawing. It is understood from the same drawingthat in order to reduce the peak current to 300 mA or less, for example,the switch MOSFETs are divided into plural form and a time difference of250 psec may be provided between their control signals. It is understoodthat a delay (i.e., an increase in tRAS) in circuit operation at thistime is controlled to 200 psec. Since the tRAS falls within a range from40 ns to 50 ns, the delay in circuit operation due to the division ofthe switch MOSFETs in the plural form and the rise in the timedifference is nothing but 0.5% thereof. It is thus understood from theinvention of the present application that the peak current can becontrolled while maintaining the speeding up of the circuit operation.

FIG. 19 is a circuit diagram showing another embodiment of the presentinvention. A circuit formed by connecting inverters in cascade form isshown in the same drawing as an internal circuit by way of illustrativeexample. The first-stage inverter is supplied with a low-level inputsignal upon deactivation. The output of the inverter is brought to ahigh level (H) and the outputs of the inverters subsequent to thisinverter are successively brought to a low level (L), a high level and alow level. Therefore, the inverters associated with the high levels ofthe outputs are electrically connected to a subground line VST and theinverters associated with the low levels of the outputs are electricallyconnected to the sub power line VCT.

A P channel switch MOSFET MC is provided between the sub power line VCTand a power line VCC and is switch-controlled by a control signal φB. AnN channel switch MOSFET MS is provided between the subground line VSTand a ground line VSS and is switch-controlled by a control signal φT.In the present embodiment, a short-circuit N channel MOSFET MT isprovided between the sub power line VCT and the subground line VST. TheMOSFET MT is switch-controlled by a control signal PT.

FIG. 20 is a timing chart for describing the operation of the embodimentdescribed above. When the internal circuit changes from an active stateto an inactive state, the control signal φB changes from a low level toa high level and the control signal φT changes from a high level to alow level. As a result, the switch MOSFETs MC and MS are shifted from anon state to an off state. In synchronism with this, the control signalPT is temporarily rendered high in level so that the switch MOSFET MT isturned on. Consequently, the sub power line VCT and the sub ground lineVST are short-circuited so as to reach an intermediate potential,whereby power consumption can be reduced.

When the short-circuit MOSFET MT is not provided, the charge stored inthe parasitic capacitance of the sub power line VCT is dischargedthrough the turned-on N channel MOSFET of the inverter whoseon-deactivation output is low in level, when the sub power line VCT andthe subground VST are shifted from an on-activation voltage to anon-deactivation voltage. Conversely, the parasitic capacitance of thesubground line VST is charged through the turned-on P channel MOSFET ofthe inverter whose on-deactivation output is high in level. Thedischarge and charge currents are used as currents to be used up. On theother hand, when the short-circuit MOSFET MT is provided, the sub powerline VCT and the subground line VST can be varied to a predeterminedpotential required to reduce the subthreshold leakage current, with acharged share between their parasitic capacitances, in other words,without a special current consumption.

The pulse width of the control signal PT for switch-controlling theshort-circuit MOSFET MT is set so that the voltage on each of the subpower line VCT and the subground line VST reaches just theon-deactivation voltage. More specifically, when the parasiticcapacitance is 200 pF, the pulse width of the control signal and thegate width of the switch MOSFET may be 100 ns and 10 μm respectively.

When the short-circuit MOSFET is not provided, voltage transitions ofthe sub power line VCT and the subground line VST need 100 μs because ofthe occurrence of a charge and discharge due to the subthreshold leakagecurrent. On the other hand, when the short-circuit MOSFET MT is used,the voltage transition can be completed in 100 ns.

In the above-described embodiment, in order to reduce the subthresholdleakage current by the threshold-voltage reduction, the logic gates andthe inverters whose on-deactivation outputs are high in level, areelectrically connected to the power line VCC and the ground sides areelectrically connected to the subground line VST. In this condition, theswitch MOSFETs connected to such a subground line VST are turned off.Further, the logic gates and the inverters whose on-deactivation outputsare low in level, are electrically connected to the ground line VSS andthe power sides are electrically connected to the sub power line VCT. Inthis condition, the switch MOSFETs connected to such a sub power lineVCT are turned off.

The above-described embodiment is characterized in that when theabove-described switches are changed from the on state to the off state,they are divided in plural form and supplied with the delayed signals soas to operate in a domino or stepwise fashion in order to reduce thedrive currents of the switch MOSFETs each having the relatively largegate capacitance to obtain the desired on resistance and reduce the peakof the source current with the turning on of each switch MOSFET. Thismeans that each switch MOSFET provides a reduction in subthresholdleakage current by the sub power line and the subground line and has thepotential for being used as a general power switch. Namely, a largenumber of function blocks can be designed so as to be mounted on onesemiconductor substrate with developments in semiconductor technology.One digital information processing system can be realized by itself.This tendency will be expected to greatly increase in the near future.

In this case, the large number of function blocks do not need to beplaced in an operating state at all times. When, at this time, functionblocks free of the need to operate exist during a period in which apredetermined data process is being carried out, it is significant thatall the currents to be used up are cut off inclusive of the leakagecurrent, such as the subthreshold leakage current. In such a case, largenoise is not allowed to be produced in a power line when a power switchis in an on or off state if viewed from a function block placed in anoperating state. As seen from such a viewpoint, a large problem ariseswhen a power supply for other non-operated function blocks is turned offor turned on during a period in which specific function blocks placed onone semiconductor integrated circuit device are in operation.

However, the switch MOSFETs employed in the previous embodiment can beset to the on or off state so as to not cause the peak current referredto above. Namely, the switch MOSFETs according to the present inventioncan be used as switch MOSFETs for selectively supplying power to eachcircuit block formed in the semiconductor integrated circuit device.

FIG. 21 is a system diagram showing one embodiment of a one-chipmicrocomputer to which the present invention is applied. In amicrocomputer MCU employed in the present embodiment, a centralprocessing unit CPU of a stored program system, which includes anarithmetic and logic unit ALU, is used as a central component of themicrocomputer MCU. A multiplexer MULT, a memory management unit MMU anda cache memory CACHE are electrically connected to the centralprocessing unit CPU through a system bus S-BUS. An address conversiontable TLB is connected to the memory management unit MMU. Further, onthe other hand, the memory management unit MMU and the cache memoryCACHE are coupled to a cache bus C-BUS. A bus controller BSC iselectrically coupled to the cache bus C-BUS.

On the other hand, the bus controller BSC is connected to a peripheralbus P-BUS and an external bus E-BUS. Of these, connected to theperipheral bus P-BUS are peripheral device controllers such as a refreshcontroller REFC, a direct memory access controller DMAC, a timer circuitTIM, a serial communication interface SCI, a digital/analog converterD/A, an analog/digital converter A/D, etc., and a clock controller CKC.An external interface EXIF is coupled to the external bus E-BUS.

On the other hand, the refresh controller REFC, the direct memory accesscontroller DMAC, the timer circuit TIM, the serial communicationinterface SCI, the digital/analog converter D/A and the analog/digitalconverter A/D are electrically connected to an interrupt controllerINTC. The interrupt controller INTC is electrically coupled to thecentral processing unit CPU through an interrupt request signal IRQ. Aclock pulse generator CPG and a plurality of clock switches to bedescribed later are electrically coupled to the clock controller CKC. Aportable information terminal PDA, an external memory, etc. areelectrically connected to the external interface EXIF.

Also connected to the interrupt controller INTC is a real time clockcircuit RTC. The real time clock circuit RTC is supplied with a clocksignal having a stable frequency, which does not vary its frequency.Thus, the real time clock circuit RTC performs accurate time control.

The real time clock circuit RTC outputs an interrupt signal RTCI to theinterrupt controller INTC at predetermined time intervals so as togenerate an interrupt request to the central processing unit CPU atpredetermined time intervals. The interrupt controller INTC is alsosupplied with an external interrupt signal OINT through a predeterminedexternal terminal. Thus, an external device is logically coupled to thecentral processing unit CPU through the interrupt controller INTC.

In the present embodiment, the clock controller CKC includes a pluralityof control registers. The central processing unit CPU writespredetermined control data into these control registers through theperipheral bus P-BUS or reads it therefrom through the peripheral busP-BUS. The clock controller CKC selectively forms a control signalPLLON, PLLSB, C0SEL1, C0SEL2 or CKEN or the like in accordance with thecontrol data set to the respective control registers and selectivelyforms a plurality of module enable signals ADEN or the like.Incidentally, these control signals and the module enable signals areindicated by one interconnection or wire to keep the drawings from beingcumbersome. It is needless to say that the clock controller CKC may beelectrically connected to the system bus S-BUS in place of theperipheral bus P-BUS.

Now, the central processing unit CPU is activated in synchronism with asystem clock signal CK1 supplied from the clock pulse generator CPG tothereby execute a predetermined arithmetic process in accordance with acontrol program read from the cache memory CACHE, for example andcontrols and supervises respective portions of a microprocessor MPU. Atthis time, the arithmetic and logic unit ALU executes arithmetic andlogical operations as needed and the multiplier MULT executes amultiplication process. Further, the memory management unit MMU convertsa logical address outputted from the central processing unit CPU into aphysical address using an address conversion table TLB upon memoryaccess.

The cache memory CACHE is composed of a quick accessible memory. Thecache memory CACHE reads and holds programs or data or the like storedin an external memory provided outside the microprocessor MPU inpredetermined block units and contributes to a high-speed operation ofthe central processing unit CPU. The central processing unit CPU, themultiplier MULT, the memory management unit MMU and the cache memoryCACHE are activated in accordance with a system clock signal CK1 havinga relatively high frequency.

The bus controller BSC manages access of the respective peripheraldevice controllers connected to the peripheral bus P-BUS to the bus andcontrols the operation of each of these peripheral device controllers.On the other hand, the refresh controller RFC corresponding to one ofthe peripheral device controllers controls the refresh operation of thedynamic RAM (random access memory) provided as an external memory, andthe direct memory access controller DMAC supports the high-speedtransfer of data between the external memory and the cache memory CACHEor the like, for example.

The timer circuit TIM supports the management of time necessary for thecentral processing unit CPU and the serial communication interface SCIsupports the transfer of serial data between the serial communicationinterface SCI and an external communication control device or the like.Further, the analog/digital converter A/D converts an analog signalinputted from an external sensor or the like into a digital signalrepresented in predetermined bits. Reversely, the digital/analogconverter D/A converts a digital signal outputted from the centralprocessing unit CPU into a predetermined analog signal and outputs it tothe outside.

The interrupt controller INTC alternatively receives interrupt requestssent from the respective peripheral device controllers in apredetermined priority order and transfers the selected one to thecentral processing unit CPU as an interrupt request signal IRQ. Theexternal interface EXIF controls and manages the transfer of databetween the respective portions of the microcomputer MCU and theportable information terminal PDA and the external memory or the likeprovided at its outside and interfaces between these external devicesand the microcomputer MCU. The bus controller BSC and the variousperipheral device controllers are activated in synchronism with a systemclock signal cks having a relatively low frequency.

In the present embodiment, the respective portions that constitute themicrocomputer MCU, are formed into a single semiconductor integratedcircuit device LSI under predetermined layout conditions. However, theseportions thereof are designed in modules as they say and are selectivelyformed based on customer or user specifications. The microcomputer MCUemployed in the present embodiment are provided in association with eachof the plurality of modules referred to above and has a plurality ofpower switch MOSFETs selectively turned on in response to effectivelevels of their corresponding module enable signals. Such switch MOSFETsare turned off upon deactivation thereof so that current consumptionthereat is substantially brought to zero.

The digital/analog converter D/A and the analog/digital converter A/Dhave linear circuit portions respectively. Even if they are in anon-operating state, relatively large current consumption occurs. Oftenthere may be cases where they do not need to operate at all times. Thus,the current consumption at the time of the deactivation of the powerswitch MOSFETs can be brought to zero by interrupting the operatingcurrent with the power switch MOSFETs referred to above. Even in thecase of other digital circuits which cause leakage current such assubthreshold leakage current or the like, its power cutoff becomes quitemeaningful.

In a system which has been brought into high integration and speedup andrendered low in voltage using MOSFETs each brought to a low thresholdvoltage, the subthreshold leakage current presents a problem as in thedynamic RAM. It is therefore needless to say that the portions in therespective function blocks, whose on-deactivation levels are fixed, areelectrically connected to the sub power line and the subground line andthe switch MOSFETs connected thereto may be turned off so as to preventthe occurrence of such leakage current.

Operations and effects obtained from the above-described embodiments areas follows:

(1) A plurality of switch MOSFETs are provided in parallel betweeninternal power lines for a plurality of circuit blocks divided forrespective functions and respectively set so as to perform circuitoperations in response to operation control signals and a power line fordelivering an operating voltage supplied from an external terminal.These switch MOSFETs are stepwise turned on in response to controlsignals produced by successively delaying the operation control signals,so as to provide the supply of operating voltages. As a result, anadvantageous effect can be brought about in that current consumption atthe time of deactivation (non-operation) of such function blocks ormodules can be brought to zero while preventing the occurrence of a peakcurrent at the time of the on/off state of each of the switch MOSFETS.

(2) A dynamic RAM is divided into an input circuit block responsive toan input signal supplied from an external terminal, inclusive of anoperation start signal, an internal circuit block activated in responseto the signal inputted from the input circuit block, and an outputcircuit block for outputting a signal outputted from the internalcircuit block to an external terminal. A plurality of switch MOSFETs areprovided in parallel between a power line for applying an operatingvoltage supplied from an external terminal and each internal power linefor a first circuit portion in the internal circuit block, which doesnot need a storage operation upon its non-operating state. Further, theswitch MOSFETs are stepwise turned on in response to controls signalsproduced by delaying a start signal supplied through the input circuitblock in turn, so as to perform the supply of each operating voltage. Asa result, an advantageous effect can be brought about in that theoccurrence of the peak current at the time of the on/off state isavoided without sacrificing operating speed and current consumption atthe time of the deactivation (non-operation) of such each function blockcan be brought to zero.

(3) An advantageous effect can be brought about in that desired circuitfunctions can be maintained without sacrificing operating speed byregularly supplying an operating voltage to each of the input circuitblock, a second circuit portion of the internal circuit block and theoutput circuit block from the power line.

(4) The internal circuit block is composed of CMOS circuits. A firstcircuit portion of the CMOS circuits includes a circuit whose outputsignal is high in level when placed in a non-operating state, which iselectrically connected to a first internal power line corresponding to aground voltage, and a circuit whose output signal is low in level, whichis electrically connected to a second internal power line associatedwith a source potential. Internal power switch circuits each composed ofa plurality of switch MOSFETs, stepwise turned on in response to controlsignals formed by delaying the start signal in turn, are respectivelyprovided between the first internal power line and a power line andbetween the second internal power line and a ground line. As a result,an advantageous effect can be brought about in that the subthresholdleakage current can be reduced while maintaining operating speed andcontrolling peak current.

(5) An advantageous effect can be obtained in that the thresholdvoltages of P channel MOSFETs and N channel MOSFETs that constitute eachCMOS circuit can be lowered so as to reduce the subthreshold leakagecurrent while maintaining a voltage reduction and an operation speedup.

(6) The input circuit block and the output circuit block arerespectively composed of CMOS circuits. P channel MOSFETs and N channelMOSFETs constituting each CMOS circuit, and MOSFETs constituting eachinternal power switch circuit referred to above, are set so as to berelatively large in threshold voltage as compared with the P channelMOSFETs and N channel MOSFETs of each CMOS circuit constituting theinternal circuit block. As a result, an advantageous effect can beobtained in that a high-speed operation can be maintained whilesuppressing the subthreshold leakage current.

(7) The above-described threshold voltages are respectively setaccording to a MOSFET channel-length dependence. Further, each of thecounter-doped layers, which are of the conductive type similar to thesource and drain of each MOSFET and contain a low concentration ofimpurities, is formed on the surface of each channel region. Thus, anadvantageous effect can be brought about in that a high-speed operationand a reduction in peak current can be achieved.

(8) The internal circuit block is divided into a plurality of blocksaccording to its operation sequence. Further, the start signal isdelayed in synchronism with the operation sequence so as to be suppliedto each internal power switch circuit. Thus, an advantageous effect canbe brought about in that the current at the time of on/off-changeover ofeach power switch is further dispersed so that the peak current can bereduced.

(9) The input circuit block corresponds to an input circuit suppliedwith an address signal and a control signal in an address multiplexsystem. The internal circuit block comprises a memory array usingdynamic memory cells, an X-system address select circuit thereof, and aY-system address select circuit. The output circuit block is dividedinto circuits such as a data input/output circuit, according to theoperation sequence of the dynamic RAM, and the power switch MOSFETs aresuccessively controlled. As a result, an advantageous effect can beobtained in that the peak current can be reasonably reduced whilemaintaining the operating speed.

(10) The internal power switch circuit provided in the Y-system addressselect circuit is composed of one or a plurality of MOSFETs set so as toprovide the flow of an operating current necessary for the operation ofsuch a circuit. A change in control signal supplied to the gate of eachMOSFET is made slow using the fact that the time required to reach thestart of its operation is long. As a result, an advantageous effect canbe obtained in that the peak current can be reduced in a simplestructure.

(11) A short-circuit switch MOSFET, temporarily turned on when internalpower switch MOSFETs associated with the first internal power line andthe second internal power line are turned off, is provided between thefirst internal power line and the second internal power line. Thus,since a charge share between the first and second internal power linesallows high-speed determination of voltages at their deactivation, anadvantageous effect can be brought about in that a further reduction insubthreshold leakage current can be achieved.

FIG. 22 is a circuit diagram illustrating a portion of the circuit shownin FIG. 14 using MOSFETS. CMOS inverters INV1, INV2, INV3, INV4, INV5, aP channel MOSFET QP60, a N channel MOSFET QN60, a SWC and an INT1 shownin FIG. 22 respectively correspond to the CMOS inverters INV1, INV2,INV3, INV4, INV5, the P channel MOSFET Q60, the N channel MOSFET QN60,the SWC and the INT1 shown in FIG. 14.

A subthreshold leakage current can be reduced by setting the thresholdvalue of a MOSFET QP60 constituting an internal power switch circuitprovided between a sub power line VCT and a power line VCC so as to berelatively larger than the threshold values of a P channel MOSFET QP62and an N channel MOSFET QN62 constituting the INV4 and making a channellength long using the dependence of a threshold value on a gate length.

The CMOS inverter INV2 shown in FIG. 22 is a circuit for controlling theinternal power switch circuit QP60 and cannot be connected to the subpower line VCT and a subground line VST.

Therefore, the channel length is made long using a gate-lengthdependence and the threshold value of the CMOS inverter INV2 is set soas to relatively larger than the threshold values of the P channelMOSFET QP62 and the N channel MOSFET QN62 constituting the INV4. As aresult, the subthreshold leakage current can be reduced.

The counter doping described with reference to FIG. 16B is used for theMOSFETs that constitute the CMOS inverters INV1, INV2, INV3, INV4 andINV5, for example. For example, the channel length of an N channelMOSFET QN61, which constitutes the CMOS inverter INV2, is made longusing the dependence of its threshold voltage on the gate length thereofso as to reduce the subthreshold leakage current. However, the thresholdvoltage of the N channel MOSFET QN61 will cause process variations.

Therefore, the counter doping described with reference to FIG. 16B isused to reduce variations in the threshold voltage due to the processvariations of the N channel MOSFET QN61. Thus, the threshold voltage ofthe N channel MOSFET QN61 can be reduced and the N channel MOSFET QN61that forms the N channel MOSFET QN61 can be activated at high speed.

The features of the present invention, have been specifically describedby way of example based on the disclosed embodiments. However, thepresent invention is not necessarily limited to the above-describedembodiments. It is needless to say that various changes can be madethereto without departing from the gist of the present invention. Amethod of forming MOSFETs each having such a threshold voltage as tosubstantially avoid a problem offered by the subthreshold leakagecurrent as in the case of, for example, the input portion, the outputcircuit and the power switch MOSFETs, can take various forms such as theutilization of the channel-length dependence, an increase in the densityof impurities at a channel portion, control on gate insulating films orthe supply of a deep back bias to a substrate with the MOSFETs formedtherein, etc.

In the internal circuit employed in the dynamic RAM, the operation modeis set by the control signal supplied from the external terminal.However, the operation mode may be determined by a command as in asynchronous dynamic RAM. In this case, the switch MOSFETs may becontrolled by a control timing circuit supplied with a command dataoutput. In a static RAM, each switch MOSFET may be controlled by a chipenable signal. However, since a circuit operating mode exists in astatic RAM for a cache memory even if an external input signal is notvaried, a switch MOS control circuit may also perform switch controlbased on a mode decision signal or the like correspondingly. The presentinvention is applicable to various semiconductor integrated circuitdevices each composed of MOSFETs as well as to the memory circuit andthe one chip microcomputer referred to above.

Advantageous effects obtained by a typical one of the features disclosedin the present application will be described in brief as follows: Aplurality of switch MOSFETs are provided in parallel between internalpower lines for a plurality of circuit blocks divided for respectivefunctions and respectively set so as to perform circuit operations inresponse to operation control signals and a power line for delivering anoperating voltage supplied from an external terminal. These switchMOSFETs are stepwise turned on in response to control signals producedby successively delaying the operation control signals, so as to providethe supply of operating voltages. As a result, current consumption atthe time of deactivation (non-operation) of such function blocks ormodules can be brought to zero while preventing the occurrence of a peakcurrent at the time of the on/off state of each switch MOSFET.

A dynamic RAM is divided into an input circuit block responsive to aninput signal supplied from an external terminal, inclusive of anoperation start signal, an internal circuit block activated in responseto the signal inputted from the input circuit block, and an outputcircuit block for outputting a signal outputted from the internalcircuit block to an external terminal. A plurality of switch MOSFETs areprovided in parallel between a power line for applying an operatingvoltage supplied from an external terminal and an internal power linefor a first circuit portion in the internal circuit block, which doesnot need a storage operation upon its non-operating state. Further, theswitch MOSFETs are turned on in a domino mode in response to controlssignals produced by delaying a start signal in successive steps in turn,so as to perform the supply of each operating voltage. As a result, theoccurrence of the peak current at the time of the on/off state of eachMOSFET can be avoided without sacrificing operating speed and currentconsumption at the time of the deactivation (non-operation) of eachfunction block can be brought to zero.

Having now fully described the invention, it will be apparent to thoseskilled in the art that many changes and modifications can be madewithout departing from the spirit or scope of the invention as set forthherein.

We claim:
 1. A semiconductor integrated circuit comprising:a pluralityof circuit blocks which perform circuit operations in response tooperation control signals, respectively; a main power line; a pluralityof sub-power lines provided for said plurality of circuit blocks,respectively; and a plurality of power switch circuits provided for saidplurality of sub-power lines, respectively, wherein each of saidplurality of power switch circuits has a plurality of switch MOSFETsprovided in parallel between said main power line and a correspondingone of said plurality of sub-power lines, and wherein said plurality ofpower switch circuits are turned on in a stepwise manner in response tosignals produced on the basis of corresponding ones of said operationcontrol signals.
 2. A semiconductor integrated circuit according toclaim 1, wherein said main power line is supplied with a ground levelpotential, and wherein said plurality of switch MOSFETs, correspondingto non-operating circuit blocks of said plurality of circuit blocks, areplaced in off states.
 3. A semiconductor integrated circuit according toclaim 2, wherein said plurality of switch MOSFETs are N channel typeMOSFETs.
 4. A semiconductor integrated circuit comprising:a plurality ofcircuit blocks which perform circuit operations in response to operationcontrol signals, respectively, a first voltage line, a plurality ofsecond voltage lines provided for said plurality of circuit blocks,respectively; a plurality of switch circuits, each of which has aplurality of switch MOSFETs being provided in parallel between saidfirst voltage line and a corresponding one of said plurality of secondvoltage lines; and a control circuit which controls said plurality ofswitch MOSFETs of said plurality of switch circuits so that saidplurality of switch circuits are turned on in a stepwise manner.
 5. Asemiconductor integrated circuit according to claim 4, wherein saidfirst voltage line is supplied with a ground level potential, andwherein said plurality of switch MOSFETs, corresponding to non-operatingcircuit blocks of said plurality of circuit blocks, are placed in offstates.
 6. A semiconductor integrated circuit according to claim 5,wherein said plurality of switch MOSFETs are N channel type MOSFETs. 7.A semiconductor integrated circuit comprising:a plurality of circuits; afirst voltage line; a plurality of second voltage lines provided forsaid plurality of circuits; a plurality of switches, each of whichincludes a plurality of switch MOSFETs having source-drain pathsprovided in parallel between said first voltage line and a correspondingone of said plurality of second voltage lines; and a control circuitwhich controls said plurality of switch MOSFETs of said plurality ofswitches so that said plurality of switches are turned on in a stepwisemanner.
 8. A semiconductor integrated circuit according to claim 7,wherein said plurality of switch MOSFETs, corresponding to adeactivating circuit of said plurality of circuits, are placed in offstates.
 9. A semiconductor integrated circuit according to claim 8,wherein said control circuit includes a delay element provided betweengates of two switch MOSFETs of said plurality of switch MOSFETs.
 10. Asemiconductor integrated circuit according to claim 9, wherein saidplurality of switch MOSFETs, corresponding to an activating circuit ofsaid plurality of circuits, are placed in on states.
 11. A semiconductorintegrated circuit according to claim 10, wherein said first voltageline is supplied with a ground level potential.
 12. A semiconductorintegrated circuit comprising:a circuit block including a plurality ofMOSFETs; and a circuit provided in order to reduce a subthresholdleakage current of said plurality of MOSFETs of said circuit block,wherein said circuit includes a first terminal connected to said circuitblock, a second terminal supplied with a first voltage, a first switchand a second switch, wherein each of said first and second switches isprovided in parallel between said first terminal and said secondterminal, and wherein said first switch and said second switch areturned on in a stepwise manner in response to a control signal.
 13. Asemiconductor integrated circuit according to claim 12, wherein saidfirst switch is a first switch MOSFET, andwherein said second switch isa second switch MOSFET.
 14. A semiconductor integrated circuit accordingto claim 13, wherein said first switch MOSFET is an N channel typeMOSFET, andwherein said second switch MOSFET is an N channel typeMOSFET.
 15. A semiconductor integrated circuit according to claim 14,wherein said first voltage is a ground potential.
 16. A semiconductorintegrated circuit according to claim 13, wherein said first switchMOSFET is a P channel type MOSFET, andwherein said second switch MOSFETis a P channel type MOSFET.
 17. A semiconductor integrated circuitaccording to claim 12, wherein said circuit block is a memory block. 18.A semiconductor integrated circuit according to claim 17, wherein saidmemory block includes a plurality of memory cells.
 19. A semiconductorintegrated circuit comprising:a first circuit including a plurality ofMOSFETS; a first line supplied with a first voltage; a second linecoupled to each of said plurality of MOSFETs; and a second circuitprovided in order to reduce a subthreshold leakage current of saidplurality of MOSFETs of said first circuit, wherein said second circuitincludes a first switch and a second switch, wherein each of said firstand second switches is provided in parallel between said first line andsaid second line, wherein said first and second switches are controlledto be in an OFF state when said first circuit is in a first state, andwherein said first and second switches are controlled to be in an ONstate when said first circuit is in a second state, with said first andsecond switches being turned on in a stepwise manner so that said secondswitch is turned on after said first switch is turned on when said firstcircuit changes from said first state to said second state.
 20. Asemiconductor integrated circuit according to claim 19, wherein saidfirst switch is a first switch MOSFET, andwherein said second switch isa second switch MOSFET.
 21. A semiconductor integrated circuit accordingto claim 20, wherein said first switch MOSFET in an N channel typeMOSFET, andwherein said second switch MOSFET is an N channel typeMOSFET.
 22. A semiconductor integrated circuit according to claim 21,further including a delay circuit coupled between the gate of said firstswitch MOSFET and the gate of said second switch MOSFET.
 23. Asemiconductor integrated circuit according to claim 20, wherein saidfirst switch MOSFET is a P channel type MOSFET, andwherein said secondswitch MOSFET is a P channel type MOSFET.
 24. A semiconductor integratedcircuit according to claim 19, wherein said first circuit is a memorycircuit.
 25. A semiconductor integrated circuit according to claim 24,wherein said memory circuit includes a plurality of memory cells.
 26. Asemiconductor integrated circuit comprising:a first circuit including aplurality of MOSFETs; a first line supplied with a first voltage; asecond line provided for said first circuit; and a second circuitprovided in order to reduce a subthreshold leakage current of saidplurality of MOSFETs of said first circuit, wherein said second circuitis coupled between said first line and said second line, and wherein, inresponse to a control signal, said first voltage is transmitted fromsaid first line to said second line by said second circuit, wherein thedriving ability of the second circuit in transmitting the first voltageto the second line increases in a stepwise manner.
 27. A semiconductorintegrated circuit according to claim 26, wherein said first circuit isa memory circuit.
 28. A semiconductor integrated circuit according toclaim 27, wherein said memory circuit includes a plurality of memorycells.
 29. A semiconductor integrated circuit according to claim 26,wherein said second circuit includes a delay circuit.
 30. Asemiconductor integrated circuit comprising:a circuit block including aplurality of MOSFETS; a first line supplied with a first voltage; asecond line coupled to each said plurality of MOSFETs; and a circuitincluding a first switch and a second switch, wherein each of said firstand second switches is provided in parallel between said first line andsaid second line, wherein said first and second switches are controlledto be in an OFF state when said circuit block is in a first state,wherein s aid first and second switches are controlled to be in an ONstate when said circuit block is in a second state, with said first andsecond switches being turned on in a stepwise manner so that said secondswitch is turned on after said first switch is turned on when saidcircuit block changes from said first state to said second state.
 31. Asemiconductor integrated circuit according to claim 30, wherein saidfirst switch and said second switch are controlled on the basis of acontrol signal.
 32. A semiconductor integrated circuit according toclaim 30,wherein said first switch is controlled by a first controlsignal, and wherein said second switch is controlled by a second controlsignal which is a delayed signal of said first control signal.
 33. Asemiconductor integrated circuit according to claim 30,wherein saidfirst state is a non-operating state of said circuit block, and whereinsaid second state is an operating state of said circuit block.
 34. Asemiconductor integrated circuit according to claim 33,wherein saidnon-operating state is a standby state of said circuit block.
 35. Asemiconductor integrated circuit according to claim 30, wherein saidfirst state is a standby state of said circuit block, andwherein saidsecond state is an operating state of said circuit block.
 36. Asemiconductor integrated circuit according to claim 30,wherein saidfirst switch is a first MOSFET, and wherein said second switch is asecond MOSFET.
 37. A semiconductor integrated circuit according to claim36,wherein said first MOSFET is an N channel type MOSFET, and whereinsaid second MOSFET is an N channel type MOSFET.
 38. A semiconductorintegrated circuit according to claim 36,wherein said first MOSFET is aP channel type MOSFET, and wherein said second MOSFET is a P channeltype MOSFET.
 39. A semiconductor integrated circuit according to claim36,wherein a signal of a first level is applied to a gate of said firstMOSFET and a gate of said second MOSFET when said circuit block is insaid first state.
 40. A semiconductor integrated circuit according toclaim 39, wherein said first level is a disable level of said first andsecond MOSFETs.
 41. A semiconductor integrated circuit according toclaim 40, wherein, when said circuit block is in said second state, saidgates of said first and second MOSFETs are applied with an enable levelsignal of said first and second MOSFETs.
 42. A semiconductor integratedcircuit according to claim 41,wherein said first MOSFET is an N channeltype MOSFET, wherein said second MOSFET is an N channel type MOSFET,wherein said first level is a low level, and wherein said second levelis a high level.
 43. A semiconductor integrated circuit comprising:afirst circuit including a plurality of MOSFETS; a first line suppliedwith a first voltage; a second line coupled to each of said plurality ofMOSFETs; and a second circuit coupled between said first line and saidsecond line, wherein said second circuit is controlled so that thecurrent path between said first line and said second line is cut offwhen said first circuit is in a non-operating state, and wherein, whensaid first circuit changes from said non-operating state to an operatingstate, said first voltage is transmitted from said first line to saidsecond line by said second circuit, wherein the driving ability of thesecond circuit in transmitting the first voltage to the second lineincreases in a stepwise manner.
 44. A semiconductor integrated circuitaccording to claim 43, wherein said second circuit is controlled on thebasis of a control signal.
 45. A semiconductor integrated circuitaccording to claim 43, wherein the second circuit is provided in orderto reduce a subthreshold leakage current of said plurality of MOSFETs ofsaid first circuit.
 46. A semiconductor integrated circuit according toclaim 43, wherein said second circuit includes a delay circuit.